Clean up RemoteTargetHandles when RecentsAnimation ends

* When in SplitScreen in overview, rotation causes
surfaces to get released, so we tell RecentsView
to null out references to them.
* Unclear why this wasn't/isn't neceessary for a
single, fullscreen task.

Fixes: 202780874
Test: Can rotate in split screen, no crash
Change-Id: Ifdfa74943945029edec37dfeeb4187616e4a23d8