Reland "Move allocation and rtp conversion logic out of payload router."

This reverts commit c2406e4eaf7703c6c64d21318186adda791e09fd.

Reason for revert: Reland by removing the conflict with the broken CL.

Original change's description:
> Revert "Move allocation and rtp conversion logic out of payload router."
> 
> This reverts commit 1da4d79ba3275b3fa48cad3b2c0949e0d3b7afe7.
> 
> Reason for revert: Need to revert https://webrtc-review.googlesource.com/c/src/+/88220
> 
> This causes a merge conflict. So need to revert this first.
> 
> Original change's description:
> > Move allocation and rtp conversion logic out of payload router.
> > 
> > Makes it easier to write tests, and allows for moving rtp module
> > ownership into the payload router in the future.
> > 
> > The RtpPayloadParams class is split into declaration and definition and
> > moved into separate files.
> > 
> > Bug: webrtc:9517
> > Change-Id: I8700628edff19abcacfe8d3a20e4ba7476f712ad
> > Reviewed-on: https://webrtc-review.googlesource.com/88564
> > Commit-Queue: Stefan Holmer <stefan@webrtc.org>
> > Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#23983}
> 
> TBR=sprang@webrtc.org,stefan@webrtc.org,srte@webrtc.org
> 
> Change-Id: I342c4bf483d975c87c706fe7f76f44e2dc60fe4c
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: webrtc:9517
> Reviewed-on: https://webrtc-review.googlesource.com/88821
> Reviewed-by: JT Teh <jtteh@webrtc.org>
> Commit-Queue: JT Teh <jtteh@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#23991}

TBR=sprang@webrtc.org,stefan@webrtc.org,srte@webrtc.org,lliuu@webrtc.org,jtteh@webrtc.org,tkchin@webrtc.org

Change-Id: I154145cdbc668feee86dbe78860147a6954fee6c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9517
Reviewed-on: https://webrtc-review.googlesource.com/89020
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23996}

BUILD.gn

@@ -422,6 +422,7 @@ if (rtc_include_tests) {
       "api:rtc_api_unittests",
       "api/audio/test:audio_api_unittests",
       "api/audio_codecs/test:audio_codecs_api_unittests",
+      "api/video/test:rtc_api_video_unittests",
       "api/video_codecs/test:video_codecs_api_unittests",
       "p2p:libstunprober_unittests",
       "p2p:rtc_p2p_unittests",