MDL-40308: Fix javascript for lti mod_form

It was toggling the state of the hidden input - not the checkbox.

mod/lti/mod_form.js

@@ -190,12 +190,10 @@
 
             var setting, control;
 
-            // Can't look these up by ID as they seem to get random IDs.
-            // Setting an id manually from mod_form made them turn into text boxes.
             var privacyControls = {
-                sendname: Y.one('input[name=instructorchoicesendname]'),
-                sendemailaddr: Y.one('input[name=instructorchoicesendemailaddr]'),
-                acceptgrades: Y.one('input[name=instructorchoiceacceptgrades]')
+                sendname: Y.one('#id_instructorchoicesendname'),
+                sendemailaddr: Y.one('#id_instructorchoicesendemailaddr'),
+                acceptgrades: Y.one('#id_instructorchoiceacceptgrades')
             };
 
             // Store a copy of user entered privacy settings as we may overwrite them