code: @RequiresApi(HONEYCOMB)
    @CheckResult @NonNull
    public Preference<Set<String>> getStringSet(@NonNull String key,
        @NonNull Set<String> defaultValue) {
      checkNotNull(key, "key == null");
      checkNotNull(defaultValue, "defaultValue == null");
      return new RealPreference<>(preferences, key, defaultValue, StringSetAdapter.INSTANCE, keyChanges);
    }
docs: Create a string set preference for {@code key} with a default of {@code defaultValue}.