Fix handling of the platform onKeyDown

Only call it if the event hasn't been handled yet.

src/components/structures/LoggedInView.tsx

@@ -485,9 +485,10 @@ class LoggedInView extends React.Component<IProps, IState> {
                     unread: true,
                 });
                 break;
-        }
+            default:
                 // if we do not have a handler for it, pass it to the platform which might
                 handled = PlatformPeg.get().onKeyDown(ev);
+        }
         if (handled) {
             ev.stopPropagation();
             ev.preventDefault();