ActivityTool: Use UnrestrictedMethod instead of ad-hoc user for process_timer

This is required cause while running  _activeSense for portal_alarm, we switch
user to nobody so as to use system user, but at the same time in proces_timer
in ActivityTool, we use the user of portal_catalog to invoke the activities.

https://lab.nexedi.com/nexedi/erp5/blob/master/product/ERP5/Document/Alarm.py#L164

The problem comes when someone runs an alarm which create a new portal_catalog,
which might not be having the required permission to invoke all the activites.
So, its better to use unrestrictedMethod or maybe, super_user here.