Cancel the back animation if the topCallback is removed or the...

Cancel the back animation if the topCallback is removed or the WindowOnBackInvokedDispatcher is detached from the window.

We'll send an onBackCancelled() event whenever the top callback is in progress and has been removed from the callbacks.

Changes:
- We send onBackCancelled() and reset the ProgressAnimator when clear() is called.
- Added a check to avoid sending an onBackInvoked() to the app if the ProgressAnimator is not in progress.

Test: atest WindowOnBackInvokedDispatcherTest
Bug: 255324784
Change-Id: I0dcc1fb869070dd5bab6c0802c933783266594e5