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
Remove GB alert drawable from reboot menu

Change-Id: I77cd3fabaed76c6cdd1ba051d628254c2f79974c