code: void setPlayingTrack(int playingTrackPosition) {
          if (playingTrackPosition < 0 || playingTrackPosition >= mSoundCloudPlaylist.getTracks().size()) {
              throw new IllegalArgumentException("No tracks a the position " + playingTrackPosition);
          }
          mCurrentTrackIndex = playingTrackPosition;
      }
docs: Allow to set the current playing song index.
  private package.
  
  @param playingTrackPosition current playing song index.