IMF cleanups, nits and nullability fixes 1/n

This clarifies the nullability of some fields, and removes unnecessary
null checks, related to ClientController, ClientState, ImeBindingState,
SessionState and AccessibilitySessionState.

Additionally explicilty marks the fallbackInputConnection fields. This
also removed checks around the nullability of the
IInputMethodClientInvoker in IMMS, as this is always a non-null value.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest FrameworksInputMethodSystemServerTests
Change-Id: Ib4de661c1a55eb3b618659b21ab91a20d76ffde6