Multiuser support for notifications, take 1.

Notifications for background users are hidden from the
panel and status bar.

Still need to add a concept of "any user" notifications (for
things coming from the system) and notification visibility
controls (for access to icons + possibly masked text of
a background user's notifications).

Change-Id: Iba121f35a6c25c2e1c44db029d776a5a59a6a008