Use the correct groupKey when canceling group child notifications

 Add 2 GroupChildCheckers:
 - Use the current group key when canceling regular/autogroup children
 - Use the original group key when canceling children of sparse groups that were aggregated (force-group singletons)

Flag: android.service.notification.notification_force_grouping
Flag: com.android.server.notification.notification_force_group_singletons
Test: atest NotificationManagerServiceTest
Bug: 357844384
Bug: 353929341
Change-Id: I63f52ddc5aae1307672adfdf72cc4675a88be092