Make Firebase dependency optional

[infertux]

This wallet crashes upon launch when Google services like Google Play and Firebase are missing. This means privacy-conscious users running LineageOS or Replicant cannot use this app.

It would be great if the Firebase and Google Analytics dependencies were made optional. This would also allow the wallet to be submitted to the F-Droid store making it available to a larger userbase.

Here's the output from adb logcat:

06-24 10:27:50.293 16786 16803 E FirebaseInstanceId: Google Play services missing or without correct permission.
06-24 10:27:50.303 16786 16804 E FirebaseCrash: Failed to load crash reporting
06-24 10:27:50.303 16786 16804 E FirebaseCrash: com.google.android.gms.internal.zzdzm: com.google.android.gms.dynamite.DynamiteModule$zzc: No acceptable module found. Local version is 0 and remote version is 0.
[...]
06-24 10:27:50.722 16786 16855 E AndroidRuntime: FATAL EXCEPTION: pool-5-thread-1
06-24 10:27:50.722 16786 16855 E AndroidRuntime: Process: com.bitcoin.mwallet, PID: 16786