code: @CheckResult @NonNull
    public Preference<Boolean> getBoolean(@NonNull String key, @NonNull Boolean defaultValue) {
      checkNotNull(key, "key == null");
      checkNotNull(defaultValue, "defaultValue == null");
      return new RealPreference<>(preferences, key, defaultValue, BooleanAdapter.INSTANCE, keyChanges);
    }
docs: Create a boolean preference for {@code key} with a default of {@code defaultValue}.