Trigger notification force grouping check on group child removal

 Fixes the scenario where an app cancels children notifications only, leaving a singleton group or summary-only group.
 Triggers autogrouping after some delay, so that the app has a chance to cancel/post notifications and create a valid group.
 Only the group summary is used to check for invalid grouping, as the other scenarios are covered by the existing implementation.

Flag: android.service.notification.notification_force_grouping

Test: atest NotificationManagerServiceTest
Test: atest GroupHelperTest

Bug: 380004477
Bug: 379467923
Change-Id: Id3f8c7cf27962699c31c75d37e971bdf86017768