$options['_expire'] in JSession expects minutes not seconds

There is a mismatch between the factory (which is converting session expiration time to seconds when it creates a session object) and JSession where the property is described as measured in minutes in several places.

https://github.com/joomla/joomla-platform/blob/staging/libraries/joomla/session/session.php#L37

https://github.com/joomla/joomla-platform/blob/staging/libraries/joomla/session/session.php#L196

This proposes to use minutes consistently.