Fix crash when adding a notification to a group while it is animating away.

This crash happens more often with the new pipeline where rows are removed and re-added quite often (rather than being set to GONE when redacted by visibility settings), and such removals also occasionally result in moving between the NSSL (which has disappear animations) and NotificationChildrenContainer.

Fixes: 210164657
Test: manual
Change-Id: I669d086a957b87762bfcb6b1ec00271282bd53fb