NetStats: Don't update stats when screen is off

Also moved registering of broadcast receiver and settings
observer to onAttachedToWindow() and unregistering to
onDetachedFromWindow() to avoid these being leaked.

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/NetworkStatsView.java

Change-Id: I077515a0550a8268b2ec4ed09b5bf0f42315733d