Add thread annotations to parts of ACMGenericCodec

This change adds annotations to all member variables that could be
annotated without acquiring any new locks, or changing the lock
structure in any other way.

BUG=3041
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18539004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6441 4adac7df-926f-26a2-2b94-8c16560cd09d