Regroup into original group when unbundling/unclassifying

 Handle regrouping when the original summary is present when:
  - unbundling == bundle channel importance changes
  - unclassifying == original channel is restored

 Also: treat bundle channels as separate sections only if importance < IMPORTANCE_DEFAULT.

Flag: android.service.notification.notification_regroup_on_classification

Test: atest NotificationManagerServiceTest
Test: atest GroupHelperTest

Bug: 382047364
Bug: 377697247
Change-Id: I7a9dad3ec15f9aa0530224c05e68c418fd7d59ac