[firebase_analytics] FirebaseAnalyticsObserver.onError should include stackTrace

[2ZeroSix]

Is your feature request related to a problem? Please describe.
It would be nice to expose StackTrace to FirebaseAnalyticsObserver.onError callback.
For example it would be useful if you want to send these errors in Crashlytics.

Describe the solution you'd like
Everything is already here, it's just not exposed in callback.