SystemUI: Separate system bar BGs for themes

This commit separates the navigation bar background colors from the
status bar background colors which allows more modular theme-ability.
Backwards compatibility for the old system bar values is still
preserved, but the new values are to be prioritized.

Thanks to Arz Bhatia for the suggestion.

Change-Id: Ibe579d16ea35ca5bdcfe9f98143c3f91cd24900f

Conflicts:
	packages/SystemUI/res/values/mahdi_colors.xml
	packages/SystemUI/src/com/android/systemui/statusbar/phone/BarTransitions.java