You can use
SuperShot, it can do screenshots in 4 different ways. See screenshot below. You want the second option, taking the shot from the current player camera position, so bypassing the UI.
The 4 buttons you see in SuperShot's plugin UI are also exposed to triggers. So you can place a UIButton atom in the scene and hook it up to that, etc.
You could also combine with Acid's
Keybindings plugin, which has a CustomTrigger feature...allowing you to set hotkeys to trigger stuff in your scene...including SuperShot. If you want to go real fancy, you can also combine with
LogicBricks, of course, build a system to automate taking screenshots from multiple perspectives, etc.
Another alternative: No idea what exactly "mirror smartphone" is or does, but if it's based on using VaM's WindowCamera, you may just be able to turn off it rendering the UI. The setting is on the "Camera" tab of the WindowCamera atom.