It's going to be a very late answer but I faced similar issue. A trick can be to call the back button event programmatically.
this.dispatchKeyEvent(new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_BACK));
As whenever you press on back button the interstitial ad is closed so firing the back button event will eventually close the interstitial ad. Rest depends on the requirement.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…