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.

PS2:
Keep track of screen on/off in a new field.

Change-Id: Ia131a0db65c4c762b7f28493e812f9fb21358a27
Merge "NetStats: Don't update stats when screen is off" into jellybean-mr2.1