Close Interstitial Ad programmatically

I wanted to close my Interstitial type ad after 30 seconds if the user did not close manually. I tried closing it by simulating the back button pressed, but it has no effect.

public void onFinish() {
  Log.d(TAG, "terminou timer pausa 30s");
  dispatchKeyEvent(new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_BACK));
  //rodarvideo();
}

Is it possible to close programmatically? If so, how do I do it?