feat(analytics): track the type of audio track

Resolves #265 by allowing the ADI team to measure videos played with the
described audio track enabled.

- add a function to indicate whether the activated audio track is audio
described or not
- add an event label named `media_audiodescription_on`

src/trackers/SRGAnalytics.js

@@ -374,6 +374,7 @@ class SRGAnalytics {
 
     if (this.isAudioTrackEnabled()) {
       labels.media_audio_track = this.getCurrentAudioTrack();
+      labels.media_audiodescription_on = this.isAudioDescriptionEnabled();
     }
 
     if (this.isTextTrackEnabled()) {
@@ -528,6 +529,19 @@ class SRGAnalytics {
     }
   }
 
+  /**
+   * Check if the audio description track is enabled.
+   *
+   * @returns {Boolean} __true__ if enabled __false__ otherwise.
+   */
+  isAudioDescriptionEnabled() {
+    const currentTrack = Array
+      .from(this.player.audioTracks())
+      .find(track => track.enabled && track.kind.includes('desc'));
+
+    return Boolean(currentTrack);
+  }
+
   /**
    * Check if the audio track is enabled.
    *