fix(Ads): Always fire CUEPOINTS_CHANGED when not using multi video el…

…ements (#7584)

This is necessary because in our UI when reusing the same video element
the unloading event is launched and we clear the cuepoints with that
event.

lib/ads/interstitial_ad_manager.js

@@ -623,6 +623,8 @@ shaka.ads.InterstitialAdManager = class {
           if (!this.baseVideo_.ended) {
             this.baseVideo_.play();
           }
+        } else {
+          this.cuepointsChanged_();
         }
         this.determineIfUsingBaseVideo_();
       }