AudioStream: Add cubeb implementation

src/common/null_audio_stream.h

@@ -4,16 +4,12 @@
 class NullAudioStream final : public AudioStream
 {
 public:
+  NullAudioStream();
   ~NullAudioStream();
 
-  static std::unique_ptr<AudioStream> Create();
-
 protected:
   bool OpenDevice() override;
   void PauseDevice(bool paused) override;
   void CloseDevice() override;
   void BufferAvailable() override;
-
-private:
-  NullAudioStream();
 };