Don't add AudioListener unless there are none in the scene

[DuFF14]

https://github.com/OSVR/OSVR-Unity/blob/master/OSVR-Unity/Assets/OSVRUnity/src/DisplayController.cs#L265

if (vrViewer.GetComponent<AudioListener>() == null)
{
     vrViewer.AddComponent<AudioListener>(); //add an audio listener
}

should be

if (FindObjectOfType<AudioListener>() == null)
{
     vrViewer.AddComponent<AudioListener>(); //add an audio listener
}

There is a memory leak if two audio listeners are in the scene.