Make timer toggle accessible

lms/static/sass/course/layout/_courseware_preview.scss

@@ -103,10 +103,10 @@
         .exam-timer-clock {
           background-color: transparent;
           border: 1px solid $white;
-          a, b {
+          #toggle_timer, b {
             color: $white;
           }
-          a {
+          #toggle_timer {
             // Overrides:
             border-top: none;
             border-right: none;
@@ -152,7 +152,7 @@
       background-color: $gray-l3;
       border-radius: 3px;
 
-      a {
+      #toggle_timer {
         @extend .exam-button-turn-in-exam;
 
         //Overrides:

lms/templates/courseware/proctored-exam-status.underscore

@@ -24,7 +24,9 @@
                 <b>
                 </b>
             </span>
-            <a id="toggle_timer" href="#" title="<%- gettext("Hide Timer") %>"><i class="fa fa-eye-slash" aria-hidden="true"></i></a>
+            <button id="toggle_timer" title="<%- gettext("Hide Timer") %>" aria-label="<%- gettext("Hide Timer") %>">
+                <i class="fa fa-eye-slash" aria-hidden="true"></i>
+            </button>
         </span>
     </div>
 </div>