MDL-36580 backup: Use the encrypted_final_element for lti "secrets"

mod/lti/backup/moodle2/backup_lti_stepslib.php

@@ -84,6 +84,8 @@ class backup_lti_activity_structure_step extends backup_activity_structure_step
             'showdescriptionlaunch',
             'icon',
             'secureicon',
+            new encrypted_final_element('resourcekey'),
+            new encrypted_final_element('password'),
             )
         );