When creating an answer, takes the codec preference from the offer.

This change is based on RFC3264:

"Although the answerer MAY list the formats in their desired order of preference, it is RECOMMENDED that unless there is a specific reason, the answerer list formats in the same relative order they were present in the offer."

BUG=2868
TEST=unit tests and manually with munge-sdp test
R=juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/14589004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6514 4adac7df-926f-26a2-2b94-8c16560cd09d