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

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

Change-Id: Ib06a713e6d93649ae8c8e5be5fd62c4f44a48416

Test Plan: WIP, don't merge yet

Reviewers: synergy, DaMadOne

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D612

Conflicts:

	packages/SystemUI/res/values/dimens.xml
	packages/SystemUI/res/values/styles.xml