Block plugin forms can no longer access 'block_theme' from form state

Drupal core was providing a 'block_theme' key in form state for block plugins. This was unreliable, and resulted in bugs when it was used, so it is no longer available in form state.

Before

$theme = $form_state->get('block_theme');

After

$theme = \Drupal::config('system.theme')->get('default');