AssetManager2: Provide a method for iterating over packages

  // Creates a new Theme from this AssetManager.

  std::unique_ptr<Theme> NewTheme();

  template <typename Func>

  void ForEachPackage(Func func) {

    for (const PackageGroup& package_group : package_groups_) {

      func(package_group.packages_.front()->GetPackageName(),

           package_group.dynamic_ref_table.mAssignedPackageId);

    }

  }

  void DumpToLog() const;

 private: