aacpsy: use a different metric for the spread of a band

This commit modifies 02dbed6e to use band->active_lines to better gauge how much information is contained within a single band and thus allow the perceptual noise subsitution to more accurately determine which bands to code as noise. The spread[w+g] used before this patch behaved more like a low-pass filter for PNS band_types, which could mistakingly mark some low frequency bands as noise.

Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>