Feature Request: CrashlyticsLogWriter should allow setting the FirebaseCrashlytics instance

[svenjacobs]

Currently CrashlyticsLogWriter uses FirebaseCrashlytics.getInstance() internally to obtain an instance of FirebaseCrashlytics. This call uses the default FirebaseApp instance.

However in our app we have multiple Firebase projects and therefore configure FirebaseApp manually in code. We need the possibility to pass in an instance of either FirebaseApp or FirebaseCrashlytics so that logs end up in the correct Firebase project.

One way could be that CrashlyticsCallsActual optionally takes this instance and that CrashlyticsCalls becomes a constructor argument of CrashlyticsLogWriter.

Note: This is written from the viewpoint of an Android application. iOS should probably receive this feature, too.