Deduplicate set of the rtp header extension uri constants

Bug: webrtc:7472
Change-Id: Ic0b4f2cc3374ba70a043310b5046d8bf91f0acbe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231949
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34990}

audio/channel_send.cc

diff --git a/audio/channel_send.cc b/audio/channel_send.cc
index 6fd2161..6a65655 100644
--- a/audio/channel_send.cc
+++ b/audio/channel_send.cc
@@ -708,9 +708,9 @@
   RTC_DCHECK_RUN_ON(&worker_thread_checker_);
   _includeAudioLevelIndication = enable;
   if (enable) {
-    rtp_rtcp_->RegisterRtpHeaderExtension(AudioLevel::kUri, id);
+    rtp_rtcp_->RegisterRtpHeaderExtension(AudioLevel::Uri(), id);
   } else {
-    rtp_rtcp_->DeregisterSendRtpHeaderExtension(AudioLevel::kUri);
+    rtp_rtcp_->DeregisterSendRtpHeaderExtension(AudioLevel::Uri());
   }
 }