Use WeakReference to hold target in values.

WeakHashMap holds strong references to values, so if we would like to
have them cleaned up automatically we need make sure we don't reference
the key strongly in the value.

Bug: 149918957
Test: Bubble animation looks OK.
Test: No GC root paths through PhysicsAnimator.
Test: atest PhysicsAnimator
Change-Id: I4e701bfb7f91662e18c317eaf895b5ea31e0860d