[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
heu? maybe there's an obvious reason but i don't see why you don't _replace_ the on_change_unit_amount() method by the @api.v8 version, and instead have it defined twice!? at least have the v7 version that calls the v8 version