code: @NonNull
      public static Set<String> keySet(@Nullable Bundle bundle) {
          if (bundle == null) {
              return Collections.emptySet();
          }
          return bundle.keySet();
      }
docs: Returns a set of keys in the bundle.
  @param bundle a bundle.
  @return a set of keys in the bundle. Empty set if bundle is null.
  @see android.os.Bundle#keySet()