Add setting to disable back button during game

[dancek]

This is very useful on Android 12 without dedicated buttons, as swiping from the edge of the screen generates a back button press event. Under time pressure moving pieces on the edge of the board may easily cause navigation out of the game and greatly affect the experience. I've lost a dozen blitz games like this.

With the setting enabled, a back button press simply does nothing which is strictly better. Even better would be to disable the swiping gestures during game altogether, but I don't know of a way to do that.