Fix 4691563: Fix memory leak caused by Tweeners hanging onto references.

This fixes a bug where the animations in MultiWaveView were keeping references
to bitmaps and preventing them from being reclaimed during GC.  The solution
is two-fold:
1. When any given animation completes, it is now removed from the list of running
animators objects.
2. When the client explicitly calls reset(), we release all references to
animators and objects.

Change-Id: Ice434ed1720fe4c253b9607ef61699d41f87f777