Make audio device mocks publicly visible

Bug: webrtc:11642
Change-Id: Ic80afca85b9f5854607aa55c403f77bd5bae1c71
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/203320
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33059}

modules/audio_device/BUILD.gn

@@ -351,6 +351,7 @@ if (is_mac) {
 }
 
 rtc_source_set("mock_audio_device") {
+  visibility = [ "*" ]
   testonly = true
   sources = [
     "include/mock_audio_device.h",