Hide peer constructors

[tobrun]

To enable constructing peer objects, we expose constructors as:

  /**
   * Internal use
   *
   * @param nativePtr - pointer to native peer
   */
  public GeoJsonSource(long nativePtr) {
    super(nativePtr);
  }

These constructors have the public modifier but that isn't required when creating these peer objects from jni. Would prefer hiding them from public api as they are noted to be for internal use only.