Remove ACMCodecDB::Codec, and make the rest of ACMCodecDB private

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1423043005

Cr-Original-Commit-Position: refs/heads/master@{#10546}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d6c0f8cac19eb549c26f032386e707559313f58f
6 files changed