Notification API hardening: forced auto-grouping

 NotificationManagerService will post a group summary on the app's behalf
 and group notifications based on several scenarios:

  1. Groups without summary
  2. Summaries without child notifications
  3. Singleton groups (summaries with single or low number of children)

Flag: android.service.notification.notification_force_grouping
Flag: com.android.server.notification.notification_force_group_singletons

Test: atest GroupHelperTest NotificationManagerServiceTest

Bug: 336488844
Change-Id: Ie1c0727c5de5e3dc55ca2a1c177d9b4660ad588d