Prevent multiple workspace state animators from being started

-> Probably an issue with the way we're wrapping ViewPropertyAnimator
   which can lead to us acting like it's valid to have multiple
   instances of a VPA. In reality I think this is very problematic.
-> For now, we can just make sure the previous animation is canceled
   if it hasn't yet completed.

Bug 18428886

Change-Id: I097eec08ec68ed098e68866fb5eda72734c51b00