fix: Prevent content from being restarted after Postroll ads (#4979)

After watching post roll ads, `this.video_.play()` was called on a completed content in the `onAdComplete` event.
The content would then restart from the beginning.

We now flag that the content is completed before we call `this.adsLoader_.contentComplete()`.
On the callback, we prevent the video element from replaying if this flag is set.

This fix is based on the Advanced Sample on the [googleads-ima-html5 repo](https://github.com/googleads/googleads-ima-html5/blob/main/advanced/ads.js#L128)

Closes #4445

lib/ads/client_side_ad_manager.js

@@ -452,7 +452,9 @@ shaka.ads.ClientSideAdManager = class {
         (new Map()).set('originalEvent', e)));
     if (this.ad_ && this.ad_.isLinear()) {
       this.adContainer_.removeAttribute('ad-active');
-      this.video_.play();
+      if (!this.video_.ended) {
+        this.video_.play();
+      }
     }
   }
 };