Annotating the rest of AudioCodingModuleImpl

A few extra locks had to be acquired as a result of the annotation.

BUG=3401
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15819004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6524 4adac7df-926f-26a2-2b94-8c16560cd09d