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.

Change-Id: I3ad4e8774ac69d1e6e1d83fa5e1ffe59b977d4e9

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java