Do not add duplicates to mWaitingVisibleActivities

processStoppingActivitiesLocked() only removes one record each time, so
instead of stopping on the next window visible (or next idle), it now
requires two such calls to move the app to destroyed, if there's only
one report, the app never gets removed.

bug: 28417200

Change-Id: I7c5eac240074ed0434a72643a8d52f1726ab906c