Implement multi-user PackageMonitor.

New APIs let you indicate what user(s) to monitor, and tell you
what user is changing when receiving a callback.

Fix package manager to only deliver package brpadcasts to the
running users.  (This isn't really a change in behavior, since
the activity manager would not deliver to stopped users anyway).
Make sure all broadcasts that package monitor receives also include
user information for it to use.

Update wallpaper service to (hopefully) now Really Correctly
monitor package changes per user.

Change-Id: Idd952dd274abcaeab452277d9160d1ae62919aa0