[FIX] account_analytic_default: computed fields cannot be readonly=False

Steps to reproduce:
- install acounting, make sure you have account_analytic_default installed
- go to settings and activate Analytic Accounting
- go to accounting > configuration > analytic accounts > create an account
- go to accounting > customers > invoices > create > make an invoice line with
an analytical account > save > edit > add a new line to the invoice and select
any product

Previous behavior:
analytic accounts previously saved are deleted by an onchange

Current behavior:
analytic accounts remains set after any update of the invoice lines

opw-2209169

closes odoo/odoo#47344

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