Oh, this is out of my expect....

This simple way to replace 'WindowManager.launch(appOrigin)' is just dispatch a event:
```
      var evt = document.createEvent('CustomEvent');
      evt.initCustomEvent('launchmusic', true, false, { origin: this.origin });
      window.dispatchEvent(evt);
```
And modify window manager as well to 
```
window.addEventListener('launchmusic', function(e) {
  setDisplayedApp(e.detail.origin);
});
```
Why this works is because whenever media control lives, the music app is alive too so you could just launch it instead of "reuse" appWindowFactory.