[FIX] mail: adapt activity view icon

Task 1889687 adds the activity view to Studio>Views, the view is represented by
icon fa-clock-o. Task 1894990 was supposed, among other things, to change the
current activity view icon (fa-th) to fa-clock-o. Since this task was not merged
it creates an inconsistency between the 'view switcher' and Studio.

Task 1902826

closes odoo/odoo#29392

addons/mail/static/src/js/views/activity/activity_view.js

@@ -13,7 +13,7 @@ var _lt = core._lt;
 var ActivityView = AbstractView.extend({
     accesskey: "a",
     display_name: _lt('Activity'),
-    icon: 'fa-th',
+    icon: 'fa-clock-o',
     config: _.extend({}, AbstractView.prototype.config, {
         Controller: ActivityController,
         Model: ActivityModel,