code: private void showToolbarNavigationIcon() {
          if (isPreferenceFragmentShown() && isNavigationIconOverridden() && !isNavigationHidden() &&
                  !(!isSplitScreen() && isButtonBarShown())) {
              displayHomeAsUp = isDisplayHomeAsUpEnabled();
              ActionBar actionBar = getSupportActionBar();
  
              if (actionBar != null) {
                  actionBar.setDisplayHomeAsUpEnabled(true);
              }
          }
      }
docs: Shows the navigation icon of the activity's toolbar.