aacenc: Rename Psy3gpp* structs to AacPsy*

This allows cleaner implementation of other psymodels using the existing
structs. It also will make it easier to interchange individual parts of
the psymodel to create hybrid models.

Patch by: Nathan Caldwell <saintdev@gmail.com>

Originally committed as revision 24890 to svn://svn.ffmpeg.org/ffmpeg/trunk