Make the RtpHeaderParserImpl available to tests and tools only.

There are a few reasons for making this test only:
* The code is only used by tests and utilities.
* The pure interface has only a single implementation so an interface isn't really needed.
  (a followup change could remove it altogether)
* The implementation always incorporates locking regardless of how the class gets used.
  See e.g. previous use in the Packet class.
* The implementation is a layer on top of RtpUtility::RtpHeaderParser which is
  sufficient for most production cases.

Change-Id: Ide6d50567cf8ae5127a2eb04cceeb10cf317ec36
Bug: none
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150658
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29010}

modules/audio_coding/BUILD.gn

@@ -1099,6 +1099,7 @@ rtc_source_set("neteq_test_tools") {
     "../../test:rtp_test_utils",
     "../rtp_rtcp",
     "../rtp_rtcp:rtp_rtcp_format",
+    "//third_party/abseil-cpp/absl/memory:memory",
     "//third_party/abseil-cpp/absl/types:optional",
   ]