Move RTP module activation into PayloadRouter.

Simplifies PayloadRouter to not accept dynamically-changing modules as
well as usage of PayloadRouter inside ViEChannel::SetSendCodec.

BUG=webrtc:5494
R=stefan@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1725363003 .

Cr-Original-Commit-Position: refs/heads/master@{#11787}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8b79b07a55a857370848724385f3d1f38c52213f
12 files changed