fix(Ads): Use region.id as id for Alternative MPD interstitial (#7671)

Author: avelad

lib/ads/interstitial_ad_manager.js

@@ -335,7 +335,7 @@ shaka.ads.InterstitialAdManager = class {
 
     /** @type {!shaka.extern.AdInterstitial} */
     const interstitial = {
-      id: null,
+      id: region.id,
       startTime: region.startTime,
       endTime: region.endTime,
       uri: alternativeMPDUri,