make Mandrill compatible with Mail system module

Ubercart and other modules work with the Mail system module. The Mandrill module can be used with Mail system but Mandrill wipes out the current settings in the mail_system variable and we have to reenter all the settings in Mail system. Mandrill could copy the approach used in Ubercart uc_store.install and other files to add and remove the Mandrill setting without wiping the other settings.

mandrill_disable(): Mandrill should check the default setting and not touch it if Mandrill is not the default. When Mandrill is the default, the code should be something like:
$mail_system = variable_get('mail_system', array('default-system' => 'DefaultMailSystem'));
if ($mail_system['default-system'] == 'MandrillMailSystem'
or $mail_system['default-system'] == 'TestingMandrillMailSystem') {
$mail_system['default-system'] == 'DefaultMailSystem';
variable_set('mail_system', $mail_system);
}

The same type of change is needed in admin submit code.