Optimize admin language detection and make it optional

Here is the full solution to this. I am not sure what additional tests will this need.

1. I made the getter return the default value if not null, so passing in an empty string default will work (the method is defined to work with a string default on the interface).
2. Used the empty string to store no preference for the user (which it is by default anyway).
3. Reworked the negotiation method as per @alexpott's suggestions from #1833010-22: Admin user language preference WSOD if ahead of path prefixes, and improved it a little bit.
3a. Now it does the admin check last.
3b. Instead of checking logged-in user, it checks admin user right away. Even less to bother with later. Also while the user may have been an admin before if the user is not an admin anymore, the preference should not take effect anymore, so current permission status is best to check.
3c. No need to check against the current language list because getAdminPreferredLangcode() does that already for us.

So its both simpler, more peformant and more correct. :) Good finds :)

This is how it looks on the UI:

Note that I thought about using the more standard "None" but figured it may be misleading here. But it may mean the same thing to you as "No preference", so we can certainly do "None" if people think its essentially the same.

With added tests for unsetting the language preference. There is already plenty test coverage for different combinations of settings and admin preference which proves we did not break things. I think this should be complete.

So any concerns left? Implemented based on suggestions form @alexpott, so assigning to him.

@alexpott: well, this mirrors getPreferredLangcode(), see:

  /**
   * Returns the preferred language code of the account.
   *
   * @param string $default
   *   (optional) Default language code to return if the account
   *   has no valid language, defaults to the site default language.
   *
   * @return string
   *   The language code that is preferred by the account.
   */
  public function getPreferredLangcode($default = NULL);

  /**
   * Returns the preferred administrative language code of the account.
   *
   * Defines which language is used on administrative pages.
   *
   * @param string $default
   *   (optional) Default language code to return if the account
   *   has no valid language, defaults to the site default language.
   *
   * @return string
   *   The language code that is preferred by the account.
   */
  public function getPreferredAdminLangcode($default = NULL);

If we are to remove the default option from getPreferredLangcode() as well, then at places like the following, we'll need to add a conditional wrapper on empty return values and a fallback on site default language from the language manager:

core/modules/action/src/Plugin/Action/EmailAction.php:      $langcode = $recipient_account->getPreferredLangcode();
core/modules/contact/src/MessageForm.php:      $recipient_langcode = $recipient->getPreferredLangcode();
core/modules/language/language.module:      $language_code = $user->getPreferredLangcode();
core/modules/update/update.fetch.inc:          $target_langcode = $target_user->getPreferredLangcode();
core/modules/user/src/AccountForm.php:    $user_preferred_langcode = $register ? $language_interface->id : $account->getPrefer...
core/modules/user/user.module:  $langcode = isset($options['langcode']) ? $options['langcode'] : $account->getPreferredLangcode();
core/modules/user/user.module:  $langcode = isset($options['langcode']) ? $options['langcode'] : $account->getPreferredLangcod();
core/modules/user/user.module:  $language = \Drupal::languageManager()->getLanguage($params['account']->getPreferredLangcode();
core/modules/user/user.module:    $langcode = $langcode ? $langcode : $account->getPreferredLangcode();

(Most of these are used in sending email).

I think removing the argument only from the admin function would be inconsistent and may end up as a WTF.

In further discussion with @alexpott, he suggested we use a boolean to decide if we fall back or not instead of letting external injection of a default. Hopefully this will look good enough now :)

Thanks!