This class is video-specific, and we want to free the name
"RtpStreamReceiver" so it can be reused for a media-independent RTP
receive class.
Also renames related files.
BUG=webrtc:7135
Review-Url: https://codereview.webrtc.org/2926253002
Cr-Commit-Position: refs/heads/master@{#18510}