[FIX] account: display currency in invoice preview

The currency symbol was not present for the fields "Untaxed Amount"
and "Total", in the online preview of an invoice.

Now the preview will use the "formatted" value for the fields.

opw-2666340

closes odoo/odoo#78744

Signed-off-by: Florian Gilbert <FlorianGilbert@users.noreply.github.com>