Fire deleteIntent of cached group summaries when all their children are gone

The posting app expects that, if the sparse group had remained, so let's keep the observable behavior.

Fixes: 393353103
Test: atest GroupHelperTest NotificationManagerServiceTest
Flag: android.service.notification.notification_force_grouping
Change-Id: Ia8a4d91cdbdee8aff24379d8443fa465fecf64f3