[FIX] hr_expense: recompute expense currency

Fix a corner case when the expense currency doesn't update when a cost is added to the product

Steps to reproduce:
- Create an expense in a foreign currency
- Add a cost to the expense product

Before fix:
Unit amount currency stays in foreign currency

After fix:
Forces expense currency to
 company currency

task-3455446

closes odoo/odoo#131043

Signed-off-by: Laurent Smet (las) <las@odoo.com>