[mythtv] Prefer lower-numbered audio stream

[J. Donavan Stanley]

Doug Larrick wrote:

>
> libavformat/libavcodec expose none of this except number of channels, 
> and even that is not properly initialized (always 2) by the time we 
> use it in avformatdecoder.cpp.  My immediate problem would be solved 
> if I fixed this bug, because it turns out the DVS stream is mono.  But 
> IMO in order to make an intelligent auto-selection, having all the 
> info outlined above would be helpful.

I know that for DVD streams we get the proper number of tracks and what 
not...