Handle back-to-back height reduction animations

We reduce the height of the notification item when the second to last
notification is dismissed (to remove the unused footer) and when the
last notification is removed (to remove the unused notification item).
If the former is runninng when the latter starts, the end result is
janky because we weren't properly cleaning up the first animation when
the second began. We now cancel the existing animation before starting
the second.

Bug: 36172954
Change-Id: I742c15b6f59874bc675b4c20a83e9578ac5e5387