Keyguard: smooth background change.

Change-Id: Ia0e4d0000db72b5cdb6fede03791805e34fa311d
Keyguard: fix statusbar lags when default widget shown.

When statusbar became expanded KeyguardStatusView receives
onKeyguardVisibilityChanged callback and refreshes ClockView.
Because it receives more than one call and every time it reads
new values from resources it cause lags in opening statusbar panel.

Replace this code with BroadcastReceiver and ContentObserver to
refresh it only in case something was changed.

Change-Id: If0dbc1db059536806852661931cb2e6436f2acf4