This commit improves the performance of product expiry activities generation.
It also removes direct refs to activity types to ensure code stability in case of user deletion of system data.
The xml refs to activity types should only be used through mail methods to ensure fallback strategies can be managed in the mail module only.

closes odoo/odoo#33300

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>