The whole point of a interstital ad is to take full screen :
"Banners are small ads that when touched typically take the user to some form of full-screen in-app browsing experience.
Interstitials, on the other hand, immediately present rich HTML5 experiences or "web apps" at natural app transition points such as launch, video pre-roll or game level load. Web apps are in-app browsing experiences with a simple close button rather than any navigation bar—the content provides its own internal navigation scheme. Interstitial ads are typically more expensive and subject to impression constraints."
You can see the rest here : InterstitalAds
You need to search for another ad provider that can do it.