Prevent the shared_secret setting from being observed when forced in config.php

When the config setting is forced eg. $CFG->bigbluebuttonbn_shared_secret = 'thesecret'; in config.php, it is possible to view this value on the settings page.

With the implementaion of https://tracker.moodle.org/browse/MDL-63734 in Moodle 3.9, if the setting was of type admin_setting_configpasswordunmask then the value would not be visible and will prevent leaking the secret.