Themes: Support theming navbar separately from SystemUI

This allows using a different theme for the navbar than the theme
applied to the rest of SystemUI.  This will not affect theming until
we have the front end in place that allows users to change this.

Credit to @BigBrother1984 for the original cherry pick

Change-Id: I753623696cdc017fc66dc5e85079fe80e5f626f9