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
Merge "Keyguard: fix statusbar lags when default widget shown." into cm-11.0