TypeInitializationException Fix for WP8 MediaPlayer

WP8's construction of media player was failing for two reasons.

1) AudioOnly flag/mode is not supported on WP8: http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj681688(v=vs.105).aspx

2) CoreWindow.GetForCurrentThread() on WP8 always returns NULL on WP8.