AudioCodingModule: Specify decoders using SdpAudioFormat

NetEq already uses SdpAudioFormat internally; this CL adds an
AudioCodingModule::RegisterReceiveCodec overload that accepts
SdpAudioFormat, and propagates it through AcmReceiver into NetEq.

The intention is to get rid of the other ways to specify decoders and
always use SdpAudioFormat. (And eventually to do the same for encoders
too.)

NOTRY=true
BUG=5801

Review-Url: https://codereview.webrtc.org/2365653004
Cr-Commit-Position: refs/heads/master@{#14506}

webrtc/modules/audio_coding/include/audio_coding_module.h

@@ -479,6 +479,11 @@ class AudioCodingModule {
   //
   virtual int32_t PlayoutFrequency() const = 0;
 
+  // Registers a decoder for the given payload type. Returns true iff
+  // successful.
+  virtual bool RegisterReceiveCodec(int rtp_payload_type,
+                                    const SdpAudioFormat& audio_format) = 0;
+
   ///////////////////////////////////////////////////////////////////////////
   // int32_t RegisterReceiveCodec()
   // Register possible decoders, can be called multiple times for