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
Merge "SystemUI: Separate system bar BGs for themes" into kitkat
SystemUI: Follow CM changes for system bars [PS3]

Tracking http://review.cyanogenmod.org/69166
For initial change ID Ibe579d16ea35ca5bdcfe9f98143c3f91cd24900f

This commit removes some hardcoded values and cleans up special casing
as suggested by people in CM. This shouldn't really change anything
for the end user or the theme creator. This only makes the codebase
easier to maintain and understand later down the line.

Change-Id: I8c70e2f1ab7ac789d8beb21a4e8ae7f32db9f015
Merge remote-tracking branch 'aospa/kitkat' into kitkat