Introduce AudioCodingModuleImpl::current_encoder_

This replaces direct reference into the codecs_ array in many places.
The variables current_send_codec_idx_ and send_codec_registered_ are
replaced.

COAUTHOR=kwiberg@webrtc.org
BUG=4228
R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47819004

Cr-Commit-Position: refs/heads/master@{#8890}
2 files changed