App's deployed on Android should be able to request a light or dark navigation bar.

It would probably make sense to expose this via the SystemChrome API although there's no comparable iOS feature that I know of. Currently SystemChrome.setSystemUIOverlayStyle() doesn't do anything on Android (see PlatformPlugin.java in the engine). Not clear if it would make sense for that API to do double-duty, since it means something different on iOS.