feat: SAF file picker, fd to jni code

app/src/main/cpp/PlaybackEngine.h

@@ -15,6 +15,7 @@ public:
     virtual ~PlaybackEngine();
     /** Play a beat sound. */
     virtual void playBeat();
+    void playMusic(int fd);
 private:
     MixingPlayer *mPlayer;
     std::string mFilesDir;