SystemUI: Fix no LED indication for missed call when screen off

When device is going to sleep, showKeyguard() will be invoked to show
lock screen. It shouldn't clear the LED notification at this moment.

Do not clear the LED notification in showKeyguard(). This should only
happen when user is actively viewing notification.

Change-Id: Id81b9e854df9510d436f6eb8185ee4411c914dd9
CRs-Fixed: 783613