D7 upgrade for layouts should leverage settings system, instead of state system.

[jenlampton]

When upgrading from Drupal 7 to Backdrop, Backdrop checks the state system to determine which layout template, and which regions, to use when placing blocks. The state system, however, is not as easy for those upgrading from Drupal 7 to leverage, as it would require a custom module that's update hooks were to execute before system_update_1025() in order to add these preferred values into the state system.

If we were to use the settings system instead, all we'd need to do in order to change the layout template used, would be to add the following to settings.php (for example):

/* Add upgrade path from theme to layouts */
$settings['upgrade_layout_name'] = 'abc';
$settings['upgrade_layout_regions'] = array(
  'header' => 'header',
  'navigation' => 'header',
  'highlighted' => 'top',
  'content' => 'content',
  'sidebar_first' => 'sidebar',
  'sidebar_second' => 'sidebar2',
  'content_top' => 'content',
  'content_bottom' => 'content',
  'bottom' => 'bottom',
  'footer' => 'footer',
);