Move RtpExtension to api/ directory and config.h/.cc to call/.

BUG=webrtc:5876
R=deadbeef@webrtc.org, solenberg@webrtc.org

Review-Url: https://codereview.webrtc.org/3004723002 .
Cr-Original-Commit-Position: refs/heads/master@{#19639}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1acbd68718476b2754a7872fb72e3a8a74166eb9
42 files changed