Network stats: Hide if there's no traffic (1/2)

Change-Id: I8b770c7ad803c063cf995e5f88c8f44e0eeae86a
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.

Change-Id: Ia131a0db65c4c762b7f28493e812f9fb21358a27