AAC encoder: in IS, fix index of sf_idx, band_type

Correct addressing of sf_idx and band_type arrays in I/S code.

Both arrays don't guarantee valid values for subwindows at all
times, depending on which coder is being used, which could result
in reading of garbage values. Instead, only the first subwindow
in the window group has to be used, ie: use w*16+g instead of
(w+w2)*16+g
