Provide Theme::setAdmin() and Theme::isAdmin()

Problem/Motivation

The helper module kindly offers an easy way to set the default theme for a site, but not a kind and helpful way to set the admin theme for a site. It would be great to offer the equivalent of setDefault() and isDefault() for the admin theme.

Proposed resolution

Something like so:

/**
   * Sets the admin theme.
   *
   * @param string $theme_name
   *   Theme name.
   *
   * @return bool
   *   TRUE if the admin theme was updated, or FALSE if the requested theme
   *   was already the admin.
   */
  public function setAdmin(string $theme_name): bool {
    if (!$this->themeHandler->themeExists($theme_name)) {
      $this->themeInstaller->install([$theme_name]);
    }
    if (!$this->isAdmin($theme_name)) {
      $this->configFactory->getEditable('system.theme')->set('admin', $theme_name)->save();
      $this->themeManager->resetActiveTheme();
      return TRUE;
    }
    return FALSE;
  }