aaccoder: move the Intensity Stereo implementation out

This commit moves the intensity stereo implementation
out from aaccoder and into a separate file. This was
possible using the previous commits.

This commit also drastically improves the IS implementation
by making it phase invariant e.g. it will always choose the
best possible phase regardless of whether M/S coding is on
or most of the coefficients have identical phases.
This also increases the quality and reduces any distortions
introduced by enablind intensity stereo.

Users are encouraged to test it out using the -aac_is 1
parameter as it has always been.

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