Remove activity from stopping activities once resumed.

Previously we were waiting for the activity to become visible before
removing from this list. This can lead to a race condition where the
activity is resumed and the stopping activities are cleaned up. If
the activity has not become visible, the resumed activity will be
stopped.

This CL addresses the issue by removing the activity from the
stopping activities list once resume has completed.

Change-Id: Ic33906327e538920961e60064a6ae976864509be
Fixes: 63804822
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackSupervisorTests#testStoppingActivityRemovedWhenResumed