Add network usage stats to status bar. [1/2]

PS2:
Get the time delta between updates and use that to show
average bytes per second.

Change-Id: Ib06a713e6d93649ae8c8e5be5fd62c4f44a48416
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
NetStats: Option to hide NetStats if there is no traffic [1/2]

Change-Id: If9626946d31f9a2ac30a3be57a182fb99ef2229f