CRM-20816 - CiviCase - Settings framework takes precedence over "Settings.xml"

CRM-20816 - CiviCase - Settings framework takes precedence over "Settings.xml"

CiviCase was originally developed before the creation of the "Settings"
framework (aka `$civicrm_settings`, `Settings API`, `civicrm_settings`,
`*.setting.php`).  At the time, it stored some settings in a special file
named "Settings.xml".

Today, this file is an anomaly which supports fewer dataflows.

With this revision:
 * For backward compatibility, the default setting is `auto`, which reads
   the setting from XML.
 * For maximum compliance with the Settings framework, the
   value from `Civi::settings()->get()` takes precedence (if defined).