Setting OPUS_SIGNAL_VOICE when enable DTX.

A better solution than forcing OPUS_APPLICATION_VOIP when enabling DTX has been found, which is to set OPUS_SIGNAL_VOICE.

This reduces the uncertainty of entering DTX over silence period of audio.

This CL contains the setup of OPUS_SIGNAL_VOICE and decoupling opus application mode with DTX.

BUG=4559
R=henrik.lundin@webrtc.org, henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46959004

Cr-Commit-Position: refs/heads/master@{#9168}
13 files changed