Add NotificationDelegate API to rebundle notification

 Allow notifications that were bundled (classified) and then unbundled to be rebundled
   => restored to their bundle channel.

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

Bug: 382047364
Change-Id: I65deccab2738126a0af4a549adc892a2346eb889