Annotating the rest of AcmGenericCodec

A few locks had to be acquired to fully annotate the class, and a few
others had to be moved.
Removing an API method that was not used.

BUG=3401
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12759004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6526 4adac7df-926f-26a2-2b94-8c16560cd09d