I have the same "issue" but this is linked to your phone settings and not the app.
In your phone settings for the screen you can select "navigationline" and set this to not use "buttons".
Then you hardly get a small white bar at the bottom. But you then need to "swipe" up from the bottom of the screen as the buttons now are virtual.
Or if the app is compiled as a "full screen app" its going to dissapear but this requires more work from zwo as google require some spesific changes compared to a regular app.