Refactor legacy pipeline group alert transfer & group manager

This fixes cases where changes to Group.suppressed or Group.alertOverride were not correctly emitted when the parent was updated before the priority child.

Bug: 185680162
Test: manual testing w/ telegram and whatsapp
Test: atest NotificationGroupAlertTransferHelperTest
Test: atest NotificationGroupManagerLegacyTest
Change-Id: I0ee8e9cf6c17a287f7ecbba6318e154ee8889174