fix(ads): Fix IMA crash when autoplay is rejected (#4518)

A preroll ad fails to start when there is no autoplay. This happens
because Shaka doesn't check if playback is allowed before calling the Ad
Manager's `start()` method. IMA's
https://github.com/googleads/googleads-ima-html5/tree/main/attempt_to_autoplay
addresses this by resolving a `play()` promise.

This waits to start AdManager until a `play` event from the video
element.

Closes #4179

lib/ads/client_side_ad_manager.js

@@ -15,7 +15,6 @@ goog.require('goog.asserts');
 goog.require('shaka.ads.ClientSideAd');
 goog.require('shaka.util.IReleasable');
 
-
 /**
  * A class responsible for client-side ad interactions.
  * @implements {shaka.util.IReleasable}
@@ -218,7 +217,11 @@ shaka.ads.ClientSideAdManager = class {
       // TODO (ismena): Need a better inderstanding of what this does.
       // The docs say it's called to 'start playing the ads,' but I haven't
       // seen the ads actually play until requestAds() is called.
-      this.imaAdsManager_.start();
+      // Note: We listen for a play event to avoid autoplay issues that might
+      // crash IMA.
+      this.video_.addEventListener('play', () => {
+        this.imaAdsManager_.start();
+      }, {once: true});
     } 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