Batch API should use the current theme to run the batches

Problem/Motivation

There is an issue for batch system that was closed(https://www.drupal.org/project/drupal/issues/539022) but i found an other related issue(https://www.drupal.org/project/drupal/issues/539022#comment-15194405) .

I am quoting this from the old issue:

There is an issue with "applies" method of BatchNegotiator class. This doesn't take the "system.batch_page.json" route into account. So for example while the default theme is used on the "system.batch_page.html" route the admin theme is used on "system.batch_page.json" route. Question is why "system.batch_page.json" route is not taken into account in the "applies" method? In my opinion this should be consistent through the whole batch process regardless of this is the batch page("system.batch_page.html") or the batch ajax calls("system.batch_page.json").

Also the following is how the theme is set for the batch at the beginning.

function batch_process($redirect = NULL, Url $url = NULL, $redirect_callback = NULL) {
  $batch =& batch_get();

  $activeThemeName = \Drupal::service('theme.manager')->getActiveTheme();
  $request = \Drupal::request();

  if (isset($batch)) {
    // Add process information
    $process_info = [
      'current_set' => 0,
      'progressive' => TRUE,
      'url' => $url ?? Url::fromRoute('system.batch_page.html'),
      'source_url' => Url::fromRouteMatch(\Drupal::routeMatch())->mergeOptions(['query' => \Drupal::request()->query->all()]),
      'batch_redirect' => $redirect,
      'theme' => \Drupal::theme()->getActiveTheme()->getName(),
      'redirect_callback' => $redirect_callback,
    ];

=>

'theme' => \Drupal::theme()->getActiveTheme()->getName(),

So this should apply to the ajax calls too.

Steps to reproduce

- Run a batch with normal path, without admin theme.
- Check which theme is used when data is processed through ajax calls in batch.

Proposed resolution

I created a patch for this issue.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet