[DO NOT MERGE] The system can post notifications for any package

The platform is able to post notifications on behalf of
any app and muting the notifications for this app should
properly work by attributing the notification not to the
system but to the app on whose behalf the notication is
posted.

Test: manual

bug:34966823

Change-Id: I4ebb2f5da1540557a8e7203b054a90ff967237f2