Samsung has something called Game Tools on the s7,s8, and s9 that automatically puts games in a letterbox mode. You have to dig into the Game Tools menu and toogle 'Full Screen' to eliminate this, in which case Game Tools asks to restart your game and restarts your game in true full screen mode if you accept. I've found after you quit the game it often turns itself back to letterbox mode. Very irritating, particularly as this often affects navigation button functionality.
With native build, I use this method to make my application full screen:
<meta-data android:name="android.max_aspect" android:value="2.1" />
How to set with Corona SDK?
PS: On config.lua file to set: scale = "zoomStrecth", When submitting to Google Play, I choose the application category, my application is full screen, but when I choose my app is game category it does not