This code shows how to change power state level when app goes in background/foreground using visibilitychange event. When app in foreground and SCREEN_NORMAL state is requested, the device does not turn off screen automatically.

document.addEventListener("webkitvisibilitychange", function() {
    	if(document.webkitHidden) {
    		tizen.power.release("SCREEN", "SCREEN_NORMAL");
    	} else {
    		tizen.power.request("SCREEN", "SCREEN_NORMAL");
    	}
    });