Regroup unbundled notification into original group

 If the original group/summary still exists, regroup unbundled notification
 into the original group.
 Otherwise move to the new section and handle as ungrouped notifications.

 Also: use the same number of ungrouped notifications (2) to trigger autogrouping
 in all cases, including regrouping because of classification.

Flag: android.service.notification.notification_regroup_on_classification
Test: atest GroupHelperTest
Bug: 377382403
Change-Id: I067e9d23b5f81f6cbc53344fb771cd4d2bb16c82