audio_processing/aecm: Create() now returns a pointer to the object

Changed Create() to return a pointer to the object rather than an error message, which is in line with how objects should be created.

BUG=441
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51939004

Cr-Commit-Position: refs/heads/master@{#9315}
5 files changed