Deflaky WindowOnBackInvokedDispatcherTest#handlesMotionEvent

The onBackProgressed callback can be triggered from
BackProgressAnimator#onBackStarted, and onMotionEvent could driven
another onBackPressed if the frame callback just happen.

Also, to test onMotionEvent in unit test, it should happen on from
Dispatcher's main thread.

Bug: 337114893
Test: atest WindowOnBackInvokedDispatcherTest#handlesMotionEvent
Change-Id: Ia7dbd591e5b8e11baf2b40b0710752ee524148d6