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

Conflicts:
	packages/SystemUI/res/layout/status_bar.xml
	packages/SystemUI/res/layout/status_bar_center_clock.xml
	packages/SystemUI/res/values/dimens.xml
	packages/SystemUI/res/values/styles.xml