Fix service_partner for contract holder change wizzard
CHANGELOG:
### Fixed
- [#701](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/701) Fix service_partner for contract holder change wizzard
Related card: https://trello.com/c/FfDHZDcj/1085-wizard-canvi-de-titular-adre%C3%A7a-de-servei
When a contract has to change its holder the service address will never change, because the service doesn't change at all. Therefore, we don't need to let it be chosen from the UI. Instead, we directly create and assign a new service typed partner_id with the old address for the new owner.
Edited by Pelayo García