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}
diff --git a/webrtc/modules/audio_coding/acm2/acm_receiver.cc b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
index 1577d2d..21dbc74 100644
--- a/webrtc/modules/audio_coding/acm2/acm_receiver.cc
+++ b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
@@ -27,6 +27,7 @@
 #include "webrtc/modules/audio_coding/neteq/include/neteq.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/system_wrappers/include/trace.h"
+#include "webrtc/modules/audio_coding/acm2/rent_a_codec.h"
 
 namespace webrtc {
 
@@ -199,7 +200,7 @@
     return *ned;
   }();
   const rtc::Optional<SdpAudioFormat> new_format =
-      RentACodec::NetEqDecoderToSdpAudioFormat(neteq_decoder);
+      NetEqDecoderToSdpAudioFormat(neteq_decoder);
 
   rtc::CritScope lock(&crit_sect_);