[FIX] sale: analytic account in confirmed state

- Activate analytic accounting
- Create a SO, validate
- Edit, and in the tab 'Additional Information' set an 'Analytic
  Account'
- Save

The analytic account is not saved.

The domain applied on the field is:
`attrs="{'readonly': ['|',('analytic_account_id','!=',False),('invoice_count','!=',0),('state','=','sale')]}"`

This means that once the account is set, the field becomes read-only.
However, a read-only field is by default not sent to the server.

opw-1882448

addons/sale/views/sale_views.xml

@@ -381,7 +381,7 @@
                                     <field name="team_id" options="{'no_create': True}"/>
                                     <field name="client_order_ref"/>
                                     <field name="company_id" options="{'no_create': True}" groups="base.group_multi_company"/>
-                                    <field name="analytic_account_id" context="{'default_partner_id':partner_invoice_id, 'default_name':name}" attrs="{'readonly': ['|',('analytic_account_id','!=',False),('invoice_count','!=',0),('state','=','sale')]}" groups="analytic.group_analytic_accounting"/>
+                                    <field name="analytic_account_id" context="{'default_partner_id':partner_invoice_id, 'default_name':name}" attrs="{'readonly': ['|',('analytic_account_id','!=',False),('invoice_count','!=',0),('state','=','sale')]}" groups="analytic.group_analytic_accounting" force_save="1"/>
                                 </group>
                                 <group name="sale_pay" string="Invoicing">
                                     <field name="date_order" attrs="{'invisible': [('state', 'in', ['sale', 'done', 'cancel'])]}"/>