Don't turn off screen in NativeDemo

For example:

```
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
application.idleTimerDisabled = true
return true
}

func applicationWillResignActive(application: UIApplication) {
application.idleTimerDisabled = false
}
}
```