Opus implementation of the AudioDecoderFactoryTemplate API

(This got reverted because of a problem with the Opus encoder parts.
Re-landing without changes.)

BUG=webrtc:7837

Review-Url: https://codereview.webrtc.org/2950453002
Cr-Original-Commit-Position: refs/heads/master@{#18855}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 96d74bb933fbe774a6324dc738c113fb5c684c0e
11 files changed