Use persistent IRCD in KeyguardUpdateMonitor to show/hide music

Now that we can have multiple IRemoteControlDisplays, create a
persistent one in KeyguardUpdateMonitor so we can more efficiently
show and hide the music controls in keyguard.

Update after review: get the current state from KeyguardUpdateMonitor
*before* the first call to showApprpriateWidgetPage().  This fixes
a flash that was sometimes seen when transitioning between orientations.

Fixes bug 7517659

Change-Id: I1b7b6f4222a99891ec06d6b6fd779dd0b86cc367