AudioDecoderOpus: Add support for 16 kHz output sample rate

In addition to the 48 kHz that we've always used.

Bug: webrtc:10631
Change-Id: If73bf7ff9c1c0d22e0d1caa245128612850f8e41
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138268
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28104}

api/audio_codecs/opus/audio_decoder_opus.h

@@ -26,7 +26,9 @@ namespace webrtc {
 // CreateAudioDecoderFactory<...>().
 struct RTC_EXPORT AudioDecoderOpus {
   struct Config {
-    int num_channels;
+    bool IsOk() const;  // Checks if the values are currently OK.
+    int sample_rate_hz = 48000;
+    int num_channels = 1;
   };
   static absl::optional<Config> SdpToConfig(const SdpAudioFormat& audio_format);
   static void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs);