audio_processing/aec: Refactors NonLinearProcessing to prepare for NEON optimizations

Puts functionality necessary to calculate sub-band coherences into a function.

BUG=3131
TESTED=trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6570 4adac7df-926f-26a2-2b94-8c16560cd09d