SystemUI: Remove the dark pixel artifacts from the back arrow drawable

The arrow had some mostly transparent pixels that made it look
kind of like the device had defective pixels. Found by Cyril Mottier.

Change-Id: Ia78453868ebe7e3291d01c96b9beafc78ea3cdcc

Conflicts:
	packages/SystemUI/res/drawable-hdpi/ic_sysbar_back.png
	packages/SystemUI/res/drawable-xhdpi/ic_sysbar_back.png
	packages/SystemUI/res/drawable-xhdpi/ic_sysbar_back_land.png