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/Channel.cc

@@ -39,7 +39,7 @@ int32_t Channel::SendData(AudioFrameType frameType,
                              ? timeStamp
                              : static_cast<uint32_t>(external_send_timestamp_);
 
-  if (frameType == kEmptyFrame) {
+  if (frameType == AudioFrameType::kEmptyFrame) {
     // When frame is empty, we should not transmit it. The frame size of the
     // next non-empty frame will be based on the previous frame size.
     _useLastFrameSize = _lastFrameSizeSample > 0;