Cancel the notification group summary when the last non-regrouped child notification is canceled

 Cache its delete intent and trigger it when all original group child notifications are canceled.

 This avoids empty summary notifications when some child notifications have their group keys overriden and the client app tracks all child notifications as being part of the original group.

Flag: EXEMPT bugfix

Test: atest GroupHelperTest
Test: atest NotificationManagerServiceTest
Bug: 422482327
Change-Id: I82e011c6bead3e456b2cdd90789861c0086b9006