[FIX] account: fiscal year end dates in wizard

- From Accounting dashboard > Configuration panel > Configure fiscal year
- Set opening date: 01/07/2018
- Set fiscal year end: June 30
- Click Apply button

An error is raised: 'Invalid fiscal year last day'.

This is because the 3 fields are linked by the constraint
`_check_fiscalyear_last_day`. However, non-stored writeable related
fields are evaluated one at a time, meaning that the constraint is
evaluated one value at a time.

We do a fancy workaround to write the 3 values at once on the company,
hence evaluating the constraint for the 3 fields once.

opw-2024927

closes odoo/odoo#34275

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>