Add lifecycle observer support for all listeners

[roxk]

Requiring users to remove listeners in onDestroy is sub-optimal now that lifecycle-aware observer is a thing.

There are two approaches:

1. Use lifecycle observer and remove the listener upon onDestroy
   • Pros
     • Less internal state is android-aware
   • Cons
     • Basically duplicating LiveData's functionality
2. Expose e.g. session state as LiveData<SessionState>
   • Pros
     • Less work. Just works™
   • Cons
     • The internal state is lifecycle aware by default. Or, we can make authgear core listen to itself and dispatch stateLiveData.value = state in onSessionStateChanged to make it fully modular

Must:

• Existing listener-based API must remain so that android apps which have not yet adopted lifecycle can still use the SDK