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.

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

Change-Id: Id43847e3d10d9d96adf2f7af1781b6978f9de975