base

Fix disappearing ripple background, treat active ripple separately

There is only a single background, and it did not correctly handle
enter() being called while exit() was busy animating. We now cancel
all animations when starting an enter or exit. Also separates the
active ripple from the list of animating (exiting) ripples.

BUG: 17042060
BUG: 17281011
Change-Id: I4d4e33560867c7c71c1bdb72b17d52d6fbd86f68