@Override
public void onStart() {
    super.onStart();
    FirebaseApp.initializeApp(this);
    mAuth = FirebaseAuth.getInstance();
    // Check if user is signed in (non-null) and update UI accordingly.
    FirebaseUser currentUser = mAuth.getCurrentUser();
    if (currentUser != null) {
        //update UI accordingly
    } else {
        //update UI accordingly

    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.android.support:design:27.1.0'
    implementation 'com.google.firebase:firebase-auth:11.6.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

apply plugin: 'com.google.gms.google-services'