Fixing issue with search bar not being visible.

- If you queue up two SearchBarDropTarget state transitions one right after
  another, the second animation will skip (since the values haven't started
  animating yet) and the first animation will complete instead, leaving the
  visibility of the search and drop target bars at odds with the current
  bar state.
- Instead, we should cancel all existing animators first, such that if we
  are already in the final state, no animations will run, and the correct
  bars will be visibile.

Bug: 23201830
Change-Id: I1f8e802821ef2b4904a3efa7edbea6ae615479b9