Call onBackCancelled() when an active callback is removed from the WindowOnBackInvokedDispatcher

When an OnBackInvokedCallback is unregistered from a WindowOnBackInvokedDispatcher, it is important to check if the callback is the current top one and if the back animation is in progress. If it is, we should call onBackCancelled() on the callback as a final step.

Test: atest WindowOnBackInvokedDispatcherTest
Bug: 276816667
Change-Id: Icaf98899c474c4e8d89d4a9c77acc309d37e9582