message: get_directory helper method
  Summary: Just make it easier to not have to pass around the unpack directory as another param. Used in later diff.
diff: @@ -180,6 +180,8 @@ class AndroidResources {
  // Classnames present in native libraries (lib/*/*.so)
  std::unordered_set<std::string> get_native_classes();
  + const std::string& get_directory() { return m_directory; }
  +
  virtual ~AndroidResources() {}
  protected:
  