[accounting] New API migration - WORK IN PROGRESS

- Task: https://www.odoo.com/web/#id=9955&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.Uy4wEA7I4G4GwPaU
ok, let's leave the refactoring remark for later.

but, what does mean the comment? if i understand correctly (and, honnestly, i hope i don't), the meaning of the comment is "i cannot overwrite the default value of company_id because the field is defined somewhere else"... geez, if it's what you wanna say then just refactor or choose which of the 2 different versions of the default value is correct but don't leave such a comment!