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/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 114cca3..ec470f8 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -18,6 +18,7 @@
 #include "webrtc/api/audio_codecs/audio_decoder_factory.h"
 #include "webrtc/api/audio_codecs/audio_format.h"
 #include "webrtc/base/constructormagic.h"
+#include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/common_types.h"  // NULL
 #include "webrtc/modules/audio_coding/codecs/cng/webrtc_cng.h"
 #include "webrtc/modules/audio_coding/neteq/audio_decoder_impl.h"