Break strong ref to ExpandableNotificationRow

Since AnimationRunner requires two GCs to be collected, one in the
binding process and then one in system ui, the source notification can
be retained for longer than expected. To avoid this, break the strong
reference to the source notification after the animation has finished
or is cancelled.

Ideally, AnimationRunner would be marked as static to avoid the outer
class from being retained longer than necessary. Unfortunately, this
requires more code changes. To keep this change focused on the largest
contributor, the notification, I'm holding off on that change.

Bug: 144109427
Test: Dumped java heap after clicking on notification to open app.
Checked that ExpandableNotificationRow wasn't retained by
AnimationRunner.

Change-Id: I69ab16cf0dff2fa96e2b0205c670e4e0d666dd14