Return a new list instead of the original list which may change

In the non-bundle ui path the returned list is stable, however, in
the bundle ui path the list is not stable. Make a copy of the list
before returning.

Flag: com.android.systemui.notification_bundle_ui
Test: manual - enable notif bundle ui
             - add a bunch of bubble notifications from the same app
               until you get a summary / group notification
             - swipe that notification
             => observe no crash happens
Bug: 401013906
Change-Id: I13077bf42ab15bb05355c66b8bb983a8b47f55cd