Precompute the AEC FFT tables, rather than initializing at run-time.

These global arrays are shared amongst all AEC instances, and were at
serious risk of data races. A Chromium TSAN bot recently caught this.

Also move the function pointer selection for optimization to
create-time. (Ideally this would only be done once.)

BUG=chromium:404133,1503
R=bjornv@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19069004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6922 4adac7df-926f-26a2-2b94-8c16560cd09d