AudioDecoder: New method SampleRateHz, + implementations for our codecs

This will let NetEq (and the factory, and every layer in between) keep
track of just the decoder, instead of decoder and sample rate.

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2024633002
Cr-Commit-Position: refs/heads/master@{#12968}

webrtc/modules/audio_coding/codecs/audio_decoder.cc

@@ -82,6 +82,10 @@ bool AudioDecoder::PacketHasFec(const uint8_t* encoded,
   return false;
 }
 
+int AudioDecoder::SampleRateHz() const {
+  return -1;
+}
+
 AudioDecoder::SpeechType AudioDecoder::ConvertSpeechType(int16_t type) {
   switch (type) {
     case 0:  // TODO(hlundin): Both iSAC and Opus return 0 for speech.