perf(Ads): Reduce latency for interstitial to start playing (#7525)

Previously we relied on an event triggered by the video itself which was
launched 4 to 60 times per second, which meant that we had an update (in
the worst case) of 250ms. Now we use a timer that is triggered every
25ms, which considerably reduces the delay.

This is important so as not to show anything from the original ad's
live.

lib/ads/interstitial_ad_manager.js

@@ -93,10 +93,25 @@ shaka.ads.InterstitialAdManager = class {
     this.lastPlayedAd_ = null;
 
     this.eventManager_.listen(this.baseVideo_, 'timeupdate', () => {
-      if (this.playingAd_ || this.basePlayer_.isRemotePlayback()) {
+      if (this.playingAd_ || this.lastTime_ ||
+          this.basePlayer_.isRemotePlayback()) {
+        return;
+      }
+      this.lastTime_ = this.baseVideo_.currentTime;
+      const currentInterstitial = this.getCurrentInterstitial_(
+          /* needPreRoll= */ true, /* numberToSkip= */ 0, this.lastPlayedAd_);
+      if (currentInterstitial) {
+        this.setupAd_(currentInterstitial, /* sequenceLength= */ 1,
+            /* adPosition= */ 1, /* initialTime= */ Date.now());
+      }
+    });
+
+    /** @private {shaka.util.Timer} */
+    this.timeUpdateTimer_ = new shaka.util.Timer(() => {
+      if (this.playingAd_ || !this.lastTime_ ||
+          this.basePlayer_.isRemotePlayback()) {
         return;
       }
-      const needPreRoll = this.lastTime_ == null || this.lastTime_ == 0;
       this.lastTime_ = this.baseVideo_.currentTime;
       // Remove last played add when the new time is before to the ad time.
       if (this.lastPlayedAd_ &&
@@ -105,12 +120,12 @@ shaka.ads.InterstitialAdManager = class {
         this.lastPlayedAd_ = null;
       }
       const currentInterstitial = this.getCurrentInterstitial_(
-          needPreRoll, /* numberToSkip= */ 0, this.lastPlayedAd_);
+          /* needPreRoll= */ false, /* numberToSkip= */ 0, this.lastPlayedAd_);
       if (currentInterstitial) {
         this.setupAd_(currentInterstitial, /* sequenceLength= */ 1,
             /* adPosition= */ 1, /* initialTime= */ Date.now());
       }
-    });
+    }).tickEvery(/* seconds= */ 0.025);
 
     /** @private {shaka.util.Timer} */
     this.pollTimer_ = new shaka.util.Timer(async () => {
@@ -219,6 +234,10 @@ shaka.ads.InterstitialAdManager = class {
     if (this.adContainer_) {
       shaka.util.Dom.removeAllChildren(this.adContainer_);
     }
+    if (this.timeUpdateTimer_) {
+      this.timeUpdateTimer_.stop();
+      this.timeUpdateTimer_ = null;
+    }
     if (this.pollTimer_) {
       this.pollTimer_.stop();
       this.pollTimer_ = null;