Do not dismiss priority children of a group being dismissed

These notifications are never visually part of the group, and should not be dismissed.  The NotificationManagerService already checks the channel for this exception, but the SystemUI NotifCollection was not.

Fixes: 287747787
Test: atest NotifCollectionTest
Merged-In: I6ee36618889b4fe1c8999ff149034a7db63fcbb7
Change-Id: I6ee36618889b4fe1c8999ff149034a7db63fcbb7