Implement music and user priority scheme in keyguard

This fixes an issue where the music transport hangs around indefinitely.
It used to get dismissed when music stopped playing and a notification came in.
This was due to a bug in JB.

Now that the bug is fixed,  the music control hangs around indefinitely until
the application actually releases it.

Since we have widget support, we can now leave the transport control in place
for as long as it's possible to resume music (state = playing | paused).

If music is playing, we start with the trasport showing
If not, and multi-user is enabled for more than one user, we show the user switcher.
Otherwise, we show the clock.

Bug 7290707

Change-Id: I6f0553a64b7b66bac7b35eca6d8a8793c15b918c