I think you need a custom AndroidManifest.xml file where you request fullscreen with no navigation bar, but I’m not 100% sure if this is enough. Anyone knows?
If you want to experiment then grab the default AndroidManifest.xml from the command line build tool, bob.jar (for Defold 1.2.75: http://d.defold.com/archive/0929102a19fd1c3d3b655a6fd4120fcc59d96ec2/bob/bob.jar). Rename to .zip, unzip it and get the AndroidManifest.xml from com/dynamo/bob/bundle/resources/android. Move the manifest file to your project and point to it in game.project. Then Google for how to set fullscreen without nav bar and make the required changes to the manifest file. Perhaps like in this SO answer: http://stackoverflow.com/a/26160995/1266551