FirebaseInstanceId.getInstance().getId()

public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {

    private static final String TAG = "MyFirebaseInsIDService";

    @Override
    public void onTokenRefresh() {
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
        Log.d(TAG, "Device Token: " + refreshedToken);
    }
}

<service
        android:name=".MyFirebaseInstanceIDService">
    <intent-filter>
        <action android:name="com.google.firebase.INSTANCE_ID_EVENT"/>
    </intent-filter>
 </service>

String deviceAppUID = Settings.Secure.getString(getContentResolver(), Settings.Secure.ANDROID_ID);
if (deviceAppUID!=null) {
    setFirebaseTokenInServer(deviceAppUID, FirebaseDeviceToken);}

DeviceTokens:
    deviceID:  firebaseDeviceToken
    deviceID:  firebaseDeviceToken