Add encoded_timestamp to AudioEncoder base class

BUG=3926
TBR=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24029004

git-svn-id: http://webrtc.googlecode.com/svn@7464 4adac7df-926f-26a2-2b94-8c16560cd09d