aacenc: disable bandtype modifying extensions when coder != twoloop

If the selected coder isn't twoloop, this commit temporarily
disables IS and PNS.
The problem is in the encode_window_bands_info() being confused
and setting invalid band_types for non-marked (normal) bands.

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