Fixes Activity cannot receive ACTION_CANCEL after onBackStarted.

Upon the client-side receiving onBackStarted, isBackGestureInProgress
returns true, and subsequent motion events are consumed by DecorView.
Once the CANCEL event is handled, the gesture is marked as intercepted,
and subsequent motion events are ignored.

Flag: com.android.window.flags.intercept_motion_from_move_to_cancel
Bug: 404173501
Test: atest OnBackInvokedCallbackGestureTest
Test: manual test with sample app, verify the CANCEL event should be
dispatched to the target which receive DOWN.

Change-Id: I8654a9d277ee38bbc483b6190332abb8833bc510