Fix Animator cancel() behavior

Previously, calling cancel() on an Animator would cause onAnimationCancel
events to be sent to all listeners. This was confusing for listeners
that were keying off this event for performing other actions, when the original
animator wasn't truly canceled (because it wasn't even running, or had already
been canceled earlier). This change hinges listener notification on
the animator actually running; no events are sent otherwise.

Also added the first set of Animator tests to verify that this behavior
is correct.

Change-Id: I81ab56559b5c0343c8dc7880e1c8235f3020975b