Add network usage stats to status bar (1/2)

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

Change-Id: I63771220333a32063f675e6fc38c9aadb6de4274
Signed-off-by: Siracuervo

Conflicts:

	packages/SystemUI/res/values/ar_dimens.xml
	packages/SystemUI/res/values/ar_styles.xml
	packages/SystemUI/res/values/dimens.xml