Reland "Move api/rtp_headers.h to its own build target."

This is a reland of a67050debcb5a3461a452a7928d7aaea1562747e

Original change's description:
> Move api/rtp_headers.h to its own build target.
>
> Reduces dependencies on the libjingle_peerconnection_api target from
> lower-level code.
>
> Bug: None
> Change-Id: I98576fc718c396cc0f720c3770acd2b696b9df89
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128565
> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#27213}

Bug: None
Tbr: kwiberg@webrtc.org
Change-Id: If15b05957e50bb8f18a33c2ed1321e672311b626
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127895
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27216}
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index 4875a54..56792e9 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -960,7 +960,7 @@
     "..:module_api",
     "..:module_api_public",
     "../../api:array_view",
-    "../../api:libjingle_peerconnection_api",
+    "../../api:rtp_headers",
     "../../api:scoped_refptr",
     "../../api/audio:audio_frame_api",
     "../../api/audio_codecs:audio_codecs_api",
@@ -1004,8 +1004,8 @@
 
   deps = [
     ":neteq",
-    "../../api:libjingle_peerconnection_api",
     "../../api:neteq_simulator_api",
+    "../../api:rtp_headers",
     "../../api/audio:audio_frame_api",
     "../../api/audio_codecs:audio_codecs_api",
     "../../rtc_base:checks",
@@ -1039,7 +1039,7 @@
   deps = [
     ":pcm16b",
     "../../api:array_view",
-    "../../api:libjingle_peerconnection_api",
+    "../../api:rtp_headers",
     "../../common_audio",
     "../../rtc_base:checks",
     "../../rtc_base:rtc_base",
@@ -1491,7 +1491,6 @@
     deps += [
       ":isac_fix",
       ":webrtc_opus",
-      "../../api:libjingle_peerconnection_api",
       "../../rtc_base:rtc_base_approved",
       "../../test:test_main",
       "../../test:test_support",