Break rtp_rtcp_format out of rtp_rtcp, to resolve circular dependencies

BUG=webrtc:8111

patch from issue 3011233002 at patchset 1 (http://crrev.com/3011233002#ps1)

Review-Url: https://codereview.webrtc.org/3014463002
Cr-Original-Commit-Position: refs/heads/master@{#19801}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 09f4481173c9bf7a96e8e7a190cf685d7fea0178