Fix cyclic deps: rent_a_codec<->audio_coding and rent_a_codec<->neteq

In short, what I did was to

  * Remove acm_common_defs.h (the stuff in it was used only by
    acm_codec_database.cc).

  * Move audio_coding_module_typedefs.h to a new build target.

  * Move the NetEqDecoder enum (and the associated
    NetEqDecoderToSdpAudioFormat function) to a new file in a new
    build target.

BUG=webrtc:7243, webrtc:7244

Review-Url: https://codereview.webrtc.org/2723253005
Cr-Commit-Position: refs/heads/master@{#17005}

webrtc/modules/audio_coding/neteq/audio_decoder_impl.h

@@ -15,7 +15,7 @@
 
 #include "webrtc/api/audio_codecs/audio_decoder.h"
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/modules/audio_coding/acm2/rent_a_codec.h"
+#include "webrtc/modules/audio_coding/neteq/neteq_decoder_enum.h"
 #include "webrtc/typedefs.h"
 
 #ifdef WEBRTC_CODEC_G722
@@ -24,8 +24,6 @@
 
 namespace webrtc {
 
-using NetEqDecoder = acm2::RentACodec::NetEqDecoder;
-
 // Returns true if |codec_type| is supported.
 bool CodecSupported(NetEqDecoder codec_type);