feat(Ads): Remove circular dependency (#6868)

To eliminate `* @Suppress {missingRequire}` it was necessary to create a
new class with the constants and deprecate the old ones. That's why this
change is feat.

build/types/ads

@@ -1,6 +1,7 @@
 # Ad Insertion Functionality
 
 +../../lib/ads/ad_manager.js
++../../lib/ads/ad_utils.js
 +../../lib/ads/ads_stats.js
 +../../lib/ads/client_side_ad.js
 +../../lib/ads/client_side_ad_manager.js

docs/tutorials/ad_monetization.md

@@ -222,7 +222,7 @@ Let's register a simple listener to Shaka's AD_STARTED event. It will log the
 start of the ad in the console.
 
 ```js
-adManager.addEventListener(shaka.ads.AdManager.AD_STARTED, () => {
+adManager.addEventListener(shaka.ads.Utils.AD_STARTED, () => {
   console.log('An ad has started');
 });
 ```
@@ -235,7 +235,7 @@ that requires access to those, here is how to get them:
 // Note that unlike in the previous example, we are capturing the AD_STARTED
 // event object here (the "e" parameter of the lambda function) so we can access
 // its properties.
-adManager.addEventListener(shaka.ads.AdManager.AD_STARTED, (e) => {
+adManager.addEventListener(shaka.ads.Utils.AD_STARTED, (e) => {
   const sdkAdObject = e['sdkAdObject'];
   const originalEvent = e['originalEvent'];
 });
@@ -250,11 +250,11 @@ Client Side or the {@link shaka.ads.AdManager#event:ImaStreamManagerLoadedEvent}
 for Server Side to get the IMA [AdManager][] or [StreamManager][] objects.
 
 ```js
-adManager.addEventListener(shaka.ads.AdManager.IMA_AD_MANAGER_LOADED, (e) => {
+adManager.addEventListener(shaka.ads.Utils.IMA_AD_MANAGER_LOADED, (e) => {
   const imaAdManager = e['imaAdManager'];
 });
 
-adManager.addEventListener(shaka.ads.AdManager.IMA_STREAM_MANAGER_LOADED, (e) => {
+adManager.addEventListener(shaka.ads.Utils.IMA_STREAM_MANAGER_LOADED, (e) => {
   const imaStreamManager = e['imaStreamManager'];
 });
 ```