Changed GetBestAudio to make detection of "best" audio stream more reliable

Example:
1. Get the highest number of channels Audio track(s).
--> Ember finds 2 x 5.1 audio tracks.
One is DTS-MA and one is DD.
2. Get the highest bit rate Audio track.
--> Ember grabs the DTS-MA Audio track and will display the icon in mainview

see: http://forum.xbmc.org/showthread.php?tid=172394&pid=1758198#pid1758198