Fix ConcurrentModificationException when dismissing bundled notifs

The iterator in `mNotifTracker` would throw a
ConcurrentModificationException when clicking done on a bundled notif.

Other code already worked with ArrayList copies but generalizing this
to the set object itself and using a java standard lib is more efficient
and less prone to similar bugs in the future.

Bug: b/389839492
Test: manual tested dismissing bundled notif
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I71f90c00c53d8423366a387df2b2d3abf7de7fb9