Base: Network traffic (1/2)

Also includes
http://gerrit.aokp.co/#/c/13356/
http://gerrit.aokp.co/#/c/13360/

@fusionjack:
* Format speed text using Formatter.formatFileSize
* Set speed text to empty if speed below 1 KB
* Don't do unnecessary things if network stats is disabled
* Address appropriate task for each intent
* SettingsObserver should only observe settings part
* Option to show/hide network speed arrow icons

Change-Id: Ie81c7c61c84f07eab58a320601fa76869821663f