"Clear all" causes auto-grouped persistent notifications losing the group summary

When an automatically generated notification group has 4+ persistent
notifications, hitting "clear all" will cause the group to lose its
automatically generated group summary notification. The solution is to
count the number of notifications belonging to a specific group as well
as the number of ongoing notifications. if the number of grouped
notifications > min before grouping limit & the group has at least one
ongoing notification, then the group summary notification will be marked
as ongoing with the flag FLAG_ONGOING_EVENT. otherwise the flag will
be removed.

Bug: 129562864
Change-Id: I65412935fbb4a74de6ac4cf159d4fd2ebe96dae8
Fix: 129562864
Test: atest NotificationManagerServiceTest