Remove DynamicAnimation when it ends.

It holds strong reference to the animation target, which is the key in
WeakHashMap so we need to make sure we clean them up when they are not
needed anymore.

Bug: 149918957
Test: Animation of bubbles look fine. This path to GC root is removed.
Change-Id: Ibdf24de730602e87b57790bb270fa7ac3524e7fe