aaccoder: tweak PNS implementation further

This commit changes a few things about the noise substitution
logic:
 - Brings back the quantization factor (reduced to 3) during
   scalefactor index calculations.
 - Rejects any zeroed bands. They should be inaudiable and it's
   a waste transmitting the scalefactor indices for these.
 - Uses swb_offsets instead of incrementing a 'start' with every
   window group size.
 - Rejects all PNS during short windows.
Overall improves quality. There was a plan to use the lfg system
to create the random numbers instead of using whatever the decoder
uses but for now this works fine. Entropy is far from important here.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>