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
Bring back the status bar activity indicators for mobile data and wifi

Google were nice enough to create xxhdpi drawables for us before removing them.

Reverts "Remove signal/wifi activity indicator icon overlays."

This reverts commit fcc901c10bffdfb9c6b31e676dec36b47d949979.

Change-Id: I7aa85f7c8442c45d17e42cac7823dfe93e3f24c1
Avoid layout passes when updating data activity indicators.

ImageView can optimize out layout passes if old and new drawable have
equal sizes. Take advantage of that by displaying a transparent drawable
with the same size as the in/out drawables when there's no activity
instead of clearing out the drawable in that case.

Needs https://gerrit.slimroms.net/3678

Change-Id: Iaaa556ffab3ed4be1da8016ed40d0a9c2efcde01