fix(Ads): Don't allow play an overlay video ad using the base video (#7740)

Author: avelad

lib/ads/interstitial_ad_manager.js

@@ -710,6 +710,10 @@ shaka.ads.InterstitialAdManager = class {
         return;
       }
     }
+    if (this.usingBaseVideo_ && interstitial.overlay) {
+      shaka.log.alwaysWarn('Unsupported interstitial', interstitial);
+      return;
+    }
     this.setupVideoAd_(interstitial, sequenceLength, adPosition, initialTime,
         oncePlayed);
   }