Update notification group mapping when group key changes

Tested by running:
"runtest -x cts/tests/app/src/android/app/cts/NotificationManagerTest.java
-m testAutogrouping_autogroupStaysUntilAllNotificationsCanceled"

The test fails before this change, and passes afterwards.

Also manually verified that the original bug that introduced this
regression (b/65100024) remains fixed.

Bug: 68215987
Change-Id: I4af198fd5257ad1dfab5e052c11f58ea84b5095a