code: @Implementation
    protected static long nativeSetup(@NonNull FileDescriptor fd, int format) throws IOException {
      FileOutputStream outputStream = fdToStream.get(fd);
  
      long potentialKey;
      do {
        potentialKey = random.nextLong();
      } while (potentialKey == 0 || outputStreams.putIfAbsent(potentialKey, outputStream) != null);
  
      nextTrackIndices.put(potentialKey, new AtomicInteger(0));
      return potentialKey;
    }
docs: Generates and returns an internal id to track the FileOutputStream corresponding to individual
  MediaMuxer instances.