SystemUI: Seperate system bar BGs for themes

This commit seperates 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
Revert "no recent apps: use white, not holo blue light."

This reverts commit a34345519f59d7609f9ee0ca56d03d9e72309809.
SystemUI: Allow themeing of the "no recents" label

This commit defines the label of the "no recent apps" label as a
simple color which allows it to be modified in themes.

Change-Id: I1346eb25dbcab519f3794d6601337ffa3b632e35