Stopped using the NetEqDecoder enum internally in NetEq.

NetEqDecoder is still used in the external interfaces, but this change
opens up the ability to use SdpAudioFormats directly, once appropriate
interfaces have been added.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2355503002
Cr-Commit-Position: refs/heads/master@{#14368}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -173,7 +173,8 @@ class NetEqImpl : public webrtc::NetEq {
 
   rtc::Optional<CodecInst> GetDecoder(int payload_type) const override;
 
-  const SdpAudioFormat* GetDecoderFormat(int payload_type) const override;
+  rtc::Optional<SdpAudioFormat> GetDecoderFormat(
+      int payload_type) const override;
 
   int SetTargetNumberOfChannels() override;