SystemUI: disable statusbar time refresh when screen off

When screen is off, don't refresh the status bar to save power.
When screen is on, refresh immediately for user experience.

Change-Id: Ie250e20bab57c096a16886680a53263c635bf6f4

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/Clock.java
Maximize widgets (1/2)

Credits to cm