Make AudioFrameType an enum class, and move to audio_coding_module_typedefs.h

Bug: webrtc:5876
Change-Id: I0c92f9410fcf0832bfa321229b3437134255dba6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128085
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27190}

modules/audio_coding/test/TestAllCodecs.cc

@@ -75,7 +75,7 @@ int32_t TestPack::SendData(AudioFrameType frame_type,
   rtp_header.payloadType = payload_type;
   rtp_header.timestamp = timestamp;
 
-  if (frame_type == kEmptyFrame) {
+  if (frame_type == AudioFrameType::kEmptyFrame) {
     // Skip this frame.
     return 0;
   }