Reimplement the builtin audio codec factories using the new stuff in api/

The whole point of all the audio codec stuff we've recently published
in api/ is to function as lego bricks so that building stuff like our
builtin audio codec factories will be easy.

(This has landed once before, but got reverted because of Chromium test
failures---apparently, someone isn't ignoring the case of the codec names
like they're supposed to. The quick fix was to preserve the same case
used by the old implementation.)

BUG=webrtc:7821, webrtc:7822

Review-Url: https://codereview.webrtc.org/2998263002
Cr-Original-Commit-Position: refs/heads/master@{#19512}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e5eb7247ffaefade251d9c8b4cc1df1530564482
15 files changed