Fix forced-grouping on notification channel updates

 Fixes some edge-cases:
    - updating a channel moves a notification to a new, empty section => no autogrouping
    - updating a channel moves a notification to a section with ungrouped notifications,
        and auto-grouping minimum count is reached => trigger autogrouping

Flag: android.service.notification.notification_force_grouping

Test: atest GroupHelperTest
Bug: 336488844
Change-Id: Ic9a07fc4dea36751cead076d8928fea55958adee