Break pipeline re-entrant call due to dismiss anim

Fixes: 224606120
Test: manual
  1. Have one clearable notification in the shade
  2. Tap "Clear All"
  3. Receive an update to the notification before the animation has
     completed
  4. Verify no crash
Change-Id: Iefd021ac6bf6ca9c959a6f410ca96f51e77142cd