Android - Make the on-screen buttons have their own entries in the Player settings menu and do not allow them to be edited

[Mimigris]

Player platform:

Android, continuous build of the Player.

Describe the issue in detail and how to reproduce it:

In the Android version of the Player, to be able to play games without physical buttons/keys, on-screen buttons are displayed and can be edited/moved/added/removed. Thing is, those buttons are mapped to keys from the actual keyboard like how it is done on other platforms, which means that, from the settings menu of the Player, it is possible to unmap or map the key corresponding to one of the on-screen buttons to something else, making it not work like what is displayed on the screen, and allowing you to be stuck with keys having no function. One way to solve this could be to have an entry specifically for the on-screen buttons that cannot be remapped or edited, making it impossible to have a scenario like that.