Don't forward KEYCODE_BACK events to app

On back navigation Activity#onUserInteraction should get called. So far
this has been ensured by sending KEYCODE_BACK events to the activity,
but marked as cancelled when enableOnBackInvokedCallback=true. This
meant that app code also saw these cancelled KeyEvents in
Activity#dispatchKeyEvent.

This CL stops this practice and registers an observer callback from
Activity.java instead to get notified on back navigation.

Bug: 436871339
Test: OnBackInvokedCallbackGestureTest
Flag: com.android.window.flags.predictive_back_stop_keycode_back_forwarding
Change-Id: Ibd20ed89d1e6086a4404bb7368160a81192e9576