fix: make sure audio track hides with one item (#4202)

src/js/control-bar/audio-track-controls/audio-track-button.js

@@ -51,6 +51,9 @@ class AudioTrackButton extends TrackButton {
    *         An array of menu items
    */
   createItems(items = []) {
+    // if there's only one audio track, there no point in showing it
+    this.hideThreshold_ = 1;
+
     const tracks = this.player_.audioTracks();
 
     for (let i = 0; i < tracks.length; i++) {