Only dispatch onPipUiStateChanged when it's not finishing

ATMS#onPictureInPictureUiStateChanged() checks the existence of
ActivityRecord before dispatching the callback, but if the
activity finishes itself, the client-side instance might be
invalid already at this point.

Bug: 325986364
Test: atest ActivityLifecyclePipTests#testDestroyPip
Change-Id: I5eb6503cb0bf3132ef4fa87360f9e45842435ad0