Add a WindowInsetsAnimationCallback to listen to IME changes.

This handles cases that were previously ignored, such as dismissing
the IME with the back key/gesture.

Removed extraneous manual state updates that were used for specific
cases previously. The only remaining state updates are from our
controlled IME animation in KeyboardInsetsHandler, as that correctly
handles the case where you start closing the IME but end up keeping
it open (or vice versa).

Bug: 277738379
Fix: 320659754
Test: Manually go through various IME transitions on Launcher,
open and close apps, lock/unlock, etc to verify keyboard state
is correctly updated. And verify b/320659754 steps do not repro.
Flag: NA

Change-Id: I4787afbb37eed815b909a0215d8afeb3894ab985