...
compileSdkVersion 25
buildToolsVersion "25.0.2"
...

...
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
<uses-permission android:name="android.permission.READ_CONTACTS" />
<uses-permission android:name="android.permission.READ_PROFILE" />
...

...
<android.support.design.widget.TextInputLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/email"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/email"
        android:inputType="textEmailAddress"
        android:maxLines="1"
        android:textColor="@android:color/white"
        />

</android.support.design.widget.TextInputLayout>
...

package com.company.product.activity;

import too.many.imports

import com.company.product.R;

import static android.Manifest.permission.READ_CONTACTS;


public class LoginActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor> {

    // UI references.
    private EditText inputPassword;
    private AutoCompleteTextView inputEmail;
    private FirebaseAuth auth;
    private ProgressBar progressBar;
    private Button btnSignup, btnLogin, btnReset;

    /**
     * Id to identity READ_CONTACTS permission request.
     */
    private static final int REQUEST_READ_CONTACTS = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        // Get Firebase auth instance
        auth = FirebaseAuth.getInstance();

        if (auth.getCurrentUser() != null) {
            startActivity(new Intent(LoginActivity.this, MainActivity.class));
            finish();
        }

        // set the content view
        setContentView(R.layout.activity_login);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        inputEmail = (AutoCompleteTextView) findViewById(R.id.email);
        inputPassword = (EditText) findViewById(R.id.password);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        btnSignup = (Button) findViewById(R.id.btn_signup);
        btnLogin = (Button) findViewById(R.id.btn_login);
        btnReset = (Button) findViewById(R.id.btn_reset_password);

        populateAutoComplete();

        //Get Firebase auth instance
        auth = FirebaseAuth.getInstance();

        btnSignup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(LoginActivity.this, SignupActivity.class));
            }
        });

        btnReset.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(LoginActivity.this, ResetPasswordActivity.class));
            }
        });

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String email = inputEmail.getText().toString();
                final String password = inputPassword.getText().toString();

                if (TextUtils.isEmpty(email)) {
                    Toast.makeText(getApplicationContext(), "Enter email address!", Toast.LENGTH_SHORT).show();
                    return;
                }

                if (TextUtils.isEmpty(password)) {
                    Toast.makeText(getApplicationContext(), "Enter password!", Toast.LENGTH_SHORT).show();
                    return;
                }

                progressBar.setVisibility(View.VISIBLE);

                //authenticate user
                auth.signInWithEmailAndPassword(email, password)
                    .addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>()
                    {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            // If sign in fails, display a message to the user. If sign in succeeds
                            // the auth state listener will be notified and logic to handle the
                            // signed in user can be handled in the listener.
                            progressBar.setVisibility(View.GONE);
                            if (!task.isSuccessful())
                            {
                                // there was an error
                                if (password.length() < 6)
                                {
                                    inputPassword.setError(getString(R.string.minimum_password));
                                }
                                else
                                {
                                    Toast.makeText(LoginActivity.this, getString(R.string.auth_failed), Toast.LENGTH_LONG).show();
                                }
                            }
                            else
                            {
                                // Send to check if the user has been verified.
                                checkIfEmailVerified();
                            }
                        }

                        //  Logic for checking if the email is verified or not
                        private void checkIfEmailVerified()
                        {
                            FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                            if (user.isEmailVerified())
                            {
                                // user is verified, finish this activity and send to MainActivity.
                                Toast.makeText(LoginActivity.this, "Successfully logged in: Lets start flying!", Toast.LENGTH_SHORT).show();
                                Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                                startActivity(intent);
                                finish();
                            }
                            else
                            {
                                // Email is not verified and prompt a message to the user and restart this activity.
                                // NOTE: don't forget to log out the user.
                                FirebaseAuth.getInstance().signOut();
                            }
                        }

                    });
            }
        });
    }

    private void populateAutoComplete() {
        if (!mayRequestContacts()) {
            return;
        }

        getSupportLoaderManager().initLoader(0, null, this);
    }

    private boolean mayRequestContacts() {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
            return true;
        }
        if (checkSelfPermission(READ_CONTACTS) == PackageManager.PERMISSION_GRANTED) {
            return true;
        }
        if (shouldShowRequestPermissionRationale(READ_CONTACTS)) {
            Snackbar.make(inputEmail, R.string.permission_rationale, Snackbar.LENGTH_INDEFINITE)
                    .setAction(android.R.string.ok, new View.OnClickListener() {
                        @Override
                        @TargetApi(Build.VERSION_CODES.M)
                        public void onClick(View v) {
                            requestPermissions(new String[]{READ_CONTACTS}, REQUEST_READ_CONTACTS);
                        }
                    });
        } else {
            requestPermissions(new String[]{READ_CONTACTS}, REQUEST_READ_CONTACTS);
        }
        return false;
    }

    /**
     * Callback received when a permissions request has been completed.
     */
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        if (requestCode == REQUEST_READ_CONTACTS) {
            if (grantResults.length == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                populateAutoComplete();
            }
        }
    }

    public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {
        return new CursorLoader(this,
                // Retrieve data rows for the device user's 'profile' contact.
                Uri.withAppendedPath(ContactsContract.Profile.CONTENT_URI,
                        ContactsContract.Contacts.Data.CONTENT_DIRECTORY), ProfileQuery.PROJECTION,

                // Select only email addresses.
                ContactsContract.Contacts.Data.MIMETYPE +
                        " = ?", new String[]{ContactsContract.CommonDataKinds.Email
                .CONTENT_ITEM_TYPE},

                // Show primary email addresses first. Note that there won't be
                // a primary email address if the user hasn't specified one.
                ContactsContract.Contacts.Data.IS_PRIMARY + " DESC");
    }

    public void onLoadFinished(Loader<Cursor> cursorLoader, Cursor cursor) {
        List<String> emails = new ArrayList<>();
        cursor.moveToFirst();
        while (!cursor.isAfterLast()) {
            emails.add(cursor.getString(ProfileQuery.ADDRESS));
            cursor.moveToNext();
        }

        addEmailsToAutoComplete(emails);
    }

    public void onLoaderReset(Loader<Cursor> cursorLoader) {

    }

    private void addEmailsToAutoComplete(List<String> emailAddressCollection) {
        //Create adapter to tell the AutoCompleteTextView what to show in its dropdown list.
        ArrayAdapter<String> adapter =
                new ArrayAdapter<>(LoginActivity.this,
                        android.R.layout.simple_dropdown_item_1line, emailAddressCollection);

        inputEmail.setAdapter(adapter);
    }


    private interface ProfileQuery {
        String[] PROJECTION = {
                ContactsContract.CommonDataKinds.Email.ADDRESS,
                ContactsContract.CommonDataKinds.Email.IS_PRIMARY,
        };

        int ADDRESS = 0;
        int IS_PRIMARY = 1;
    }

}