By default, the app works in Screen pinning mode. Users can unpin the app by holding recent and home buttons down for a while. In this case, a pin or pattern lock is required for the device.
To forbid any possible system buttons interactions, need to mark the app as device owner by the next adb command:
adb shell dpm set-device-owner com.kiosk.kiosk/.AdminReceiver
Inspired by this article.