Keyguard: don't refresh ui when screen off

KeyguardStatusView is doing refresh all the time,
which cause high power when screen off.

PatchSet:
- We turn of the refresh when screen is off which is a good
  idea. But we should force latest next refresh when screen turns
  on. Du that a timechanged refresh maybe comes a bit later.

Change-Id: Icbb9a92bae3cf099417ab820617dd3f17e49f75a