Drop KEYCODE_BACK if a callback is already receiving dispatch.

Bug: 296916226
Fixes: 296916226
Test: m -j
Test: atest OnBackInvokedCallbackGestureTest
Test: atest BackAnimationControllerTest
Test: atest WindowOnBackInvokedDispatcherTest

Change-Id: I5cb26e9858d3e5aa955f623c4e87bd37176dd373
3 files changed