Finish stopping activities if requested

If activity was in process of becoming invisible and it needed to be
finished, we weren't actually finishing it if we tried to do it while
in STOPPING state. This lead to activity being removed from stopping
list and stuck in FINISHING state.

Bug: 34195099
Test: android.server.cts.ActivityManagerActivityVisibilityTests
Test: #testFinishActivityWithMoveTaskToBackAfterPause
Test: #testFinishActivityWithMoveTaskToBackAfterStop
Change-Id: Ic892f998b7fda7d5ede9a84b1ab837dbef40af21