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: I78def75f34f43fbc9853a590745414777d37e443
Merge "NetStats: Don't update stats when screen is off" into jb-mr2