Fix back to home animation restarts on a second swipe.

If a second swipe starts before a back to home animation finishes, it's
possible for subsequent MOVE events to start another back animation.
The fix is to only start back animation when the swipe's DOWN event is not
rejected.

Bug: 240109403
Fixes: 240109403
Test: Follow repro steps in 240109403#10 and make sure a second
animation cannot be started.
Test: BackAnimationControllerTest

Change-Id: Id2a5fb00f632fb9be175ef10cad7fea511d09ae0