Remove finish-callbacks from BackProgressAnimator upon callback unregistration

Finish-callbacks need to be removed from BackProgressAnimator when an active callback is unregistered and a onBackCancelled() event is injected. Otherwise the same callback may get another onBackCancelled() or onBackInvoked() call after the injected onBackCancelled() call.

Bug: 365718226
Test: WindowOnBackInvokedDispatcherTest
Test: BackProgressAnimatorTest
Flag: EXEMPT bugfix
Change-Id: I73eb8ffa5018275807a3e778e4367fd0ed73730a