Add lifecycle observer support for all listeners

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`
- 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

Start by reviewing the existing listener-based API and the two proposed approaches: lifecycle observers that remove listeners on onDestroy, or exposing session state through LiveData. Confirm that the existing listener API remains available for apps without lifecycle support, then define the supported behavior and validation for lifecycle-aware listeners.

Written by the indexing model from the issue text.