Improve tethering notification icons

Replaces the stock icons with the ones I submitted as part of
http://review.cyanogenmod.com/#/c/15930/ in CM9.

Change-Id: Iecfae9ad579c6c9ae06da0a08b61e9359a8a37b0
Add xxhdpi versions of our improved tethering icons

Change-Id: If943f81389774a1f8ca0407cab4756687a273be7