MDL-64492 course: Hide activity icons for Ally

Empty alt text results in an image with no label.
role="presentation" does not hide - it only removes the semantic information.
If the image provides no information, make it hidden with aria-hidden.

course/renderer.php

@@ -697,7 +697,7 @@ class core_course_renderer extends plugin_renderer_base {
 
         // Display link itself.
         $activitylink = html_writer::empty_tag('img', array('src' => $mod->get_icon_url(),
-                'class' => 'iconlarge activityicon', 'alt' => ' ', 'role' => 'presentation')) .
+                'class' => 'iconlarge activityicon', 'alt' => '', 'role' => 'presentation', 'aria-hidden' => 'true')) .
                 html_writer::tag('span', $instancename . $altname, array('class' => 'instancename'));
         if ($mod->uservisible) {
             $output .= html_writer::link($url, $activitylink, array('class' => $linkclasses, 'onclick' => $onclick));