aacenc: Fix threshold-in-quiet calculation in the 3GPP psymodel.

aacenc: Fix threshold-in-quiet calculation in the 3GPP psymodel.

Removing the modification vastly improves quality (at a slight bitrate
cost) for some samples. castanets.wav is a good example. The closest
equivalent I see to the modification in the 3GPP spec is a similar
modification (over a specific frequency range) when TNS is used.

This also changes the threshold-in-quiet calculation to match the
3GPP spec.

patch by Nathan Caldwell saintdev (at) gmail

Originally committed as revision 25588 to svn://svn.ffmpeg.org/ffmpeg/trunk