fix(Ads): Initialize correctly the IMA ads manager (#5541)

Fixes #4800

Backported to v4.3.x

lib/ads/client_side_ad_manager.js

@@ -33,6 +33,9 @@ shaka.ads.ClientSideAdManager = class {
     /** @private {HTMLMediaElement} */
     this.video_ = video;
 
+    /** @private {boolean} */
+    this.videoPlayed_ = false;
+
     /** @private {ResizeObserver} */
     this.resizeObserver_ = null;
 
@@ -82,6 +85,10 @@ shaka.ads.ClientSideAdManager = class {
     this.eventManager_.listen(this.video_, 'ended', () => {
       this.adsLoader_.contentComplete();
     });
+
+    this.eventManager_.listenOnce(this.video_, 'play', () => {
+      this.videoPlayed_ = true;
+    });
   }
 
   /**
@@ -219,9 +226,14 @@ shaka.ads.ClientSideAdManager = class {
       // seen the ads actually play until requestAds() is called.
       // Note: We listen for a play event to avoid autoplay issues that might
       // crash IMA.
-      this.eventManager_.listenOnce(this.video_, 'play', () => {
+      if (this.videoPlayed_) {
         this.imaAdsManager_.start();
-      });
+      } else {
+        this.eventManager_.listenOnce(this.video_, 'play', () => {
+          this.videoPlayed_ = true;
+          this.imaAdsManager_.start();
+        });
+      }
     } catch (adError) {
       // If there was a problem with the VAST response,
       // we we won't be getting an ad. Hide ad UI if we showed it already