Fix Notification group dismissing animations

When dismissing a notification group, keep the notifiication children in their parent to ensure correct animations.

Fixes: 251058695
Fixes: 253266681
Fixes: 254277797

Bug: 259217907

Test: atest ShadeViewDifferTest
Test: atest NotifCollectionTest
Test: atest ExpandableNotificationRowTest
Test: Manually dismiss Notification Groups (both app created groups and
auto-grouped notifs)

Change-Id: Ia5b6e5a1f058b5608896e33d0db2bd5b8fa39667