Move AEC3 configuration to its own file under api/audio

This is one of several small steps of separating APM and AEC3.

Bug: webrtc:8844
Change-Id: Ib6e518fec5f7566cab3823ab35fcede8433f8f4e
Reviewed-on: https://webrtc-review.googlesource.com/53142
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22028}

api/BUILD.gn

@@ -219,6 +219,15 @@ rtc_source_set("audio_mixer_api") {
   ]
 }
 
+rtc_source_set("aec3_config") {
+  visibility = [ "*" ]
+  sources = [
+    "audio/echo_canceller3_config.h",
+  ]
+
+  deps = []
+}
+
 rtc_source_set("audio_options_api") {
   visibility = [ "*" ]
   sources = [