drupal_mail_system() now uses a factory service

drupal_mail_system(), which returns the mail system class for a given mail module and mail key combination, is now a service.

The default behavior is unchanged but this allows modules to replace the mail.factory service, giving them for example the flexibility to return adapter class that uses diferent implementations for formatting and sending an e-mail.

Update (2014-02-27): This change record is obsolete after Mail backends are now annotated plugins, which replaced mail.factory with plugin.manager.mail.