Adding "usedtx" as Opus codec parameter.

This is according to https://tools.ietf.org/html/draft-spittka-payload-rtp-opus-03

Specifically,

usedtx: specifies if the decoder prefers the use of DTX. values are 1 and 0. If no value is specified, usedtx is assumed to be 0.

BUG=1014
R=juberti@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48499004

Cr-Commit-Position: refs/heads/master@{#8872}
6 files changed