fix(Ads): Destroy preloadManagers on stop interstitials (#8268)

Author: avelad

lib/ads/interstitial_ad_manager.js

@@ -329,6 +329,18 @@ shaka.ads.InterstitialAdManager = class {
     this.interstitialIds_.clear();
     this.interstitials_.clear();
     this.player_.destroyAllPreloads();
+    if (this.preloadManagerInterstitials_.size) {
+      const values = Array.from(this.preloadManagerInterstitials_.values());
+      for (const value of values) {
+        if (value) {
+          value.then((preloadManager) => {
+            if (preloadManager) {
+              preloadManager.destroy();
+            }
+          });
+        }
+      };
+    }
     this.preloadManagerInterstitials_.clear();
     this.player_.detach();
     this.playingAd_ = false;