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: I63771220333a32063f675e6fc38c9aadb6de4274

Conflicts:
	core/java/android/provider/Settings.java
	packages/SystemUI/res/layout/status_bar.xml
	packages/SystemUI/res/values/dimens.xml
	packages/SystemUI/res/values/styles.xml

Conflicts:

	core/java/android/provider/Settings.java
	packages/SystemUI/res/values/cr_dimens.xml
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