Add NotificationDelegate API to unbundle notification

 Allow notifications that were bundled (classified) to be unbundled => restored to their original channel.
 If the original app-provided summary exists, the unbundled notification will also be regrouped back to its original group.

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

Bug: 382047364
Change-Id: I5c156b92366c4df98fbdf5e438173f2f93498c6e