Regroup app-grouped notifications after classification

 Move child notifications into new sections and regroup them
 after classification (bundling).
 The original group summaries will be canceled once the groups are empty.

Flag: android.service.notification.notification_regroup_on_classification
Test: atest GroupHelperTest
Test: atest NotificationManagerServiceTest

Bug: 372775153

Change-Id: Ifda85a486729c0493301ff79e76e9f341a093d45