[KeepScreenOn] Avoid dropping random point when touching screen to keep it on

To save smartphone battery in remote environments, one would be tempted/wise to have the screen brightness reduced, or the screen switched off, automatically after X seconds of inactivity. To switch the screen back ON/bright, one would touch the screen. But when drawing a perimeter/polygon, this automatically adds a point. And when the screen is dark, one could also easily press the backward button by mistake that would even delete the point without warning (#3592).