Currently, simple pos users can see the due amount of the customers, unless the customer wasn't loaded and we need to search for a customer that isn't loaded.

Steps to reproduce:
-------------------
* Create a new customer, make a sale order and invoice it. This will set an amount due for that customer
* Connect to the shop with a user that has the `point_of_sale.group_pos_user` but not the `account.group_account_readonly` group
* Open customer list
> Observation: You can see the amount due for multiple customers
* Search for the customer created
* Select search more
> You can't see the amount due for that customer

Why the fix:
------------
Since simple pos users are allowed to see the amount due for the loaded customer, there is no reason they shouldn't see it as well for a customer they need to load.

Commit allowing simple pos users to see (and settle) customer accounts: https://github.com/odoo/enterprise/commit/37fa4d5f4ed7c7d77f73395a53b7b3ab7006afc4

When loading the pos session `_loader_params_res_partner` is called first and later is called `_get_pos_ui_res_partner`. It is in the function `_get_pos_ui_res_partner` that the amount due is compted if the user does not belong to the group `account.group_account_readonly`.

https://github.com/odoo/enterprise/blob/a35a4755cdf86bcfeda0aca6c06397748ba27362/pos_settle_due/models/pos_session.py#L22-L28

However when we load a customer that wasn't previously loaded, only `_loader_params_res_partner` is loaded. And since the users does not belong to the group `account.group_account_readonly`, the field witll not get loaded.

We now compute the amount due with the same logic as in `_get_pos_ui_res_partner`.

opw-4141955

**Steps to reproduce:**
- Install l10n_mx_reports
- Switch to a Mexican company (e.g. ESCUELA KEMPER URGATE)
- Go to "Accounting / Configuration / Accounting / Chart of Accounts"
- Create an account:
  * Account Name: [any]
  * Code: 123456789
  * Type: Bank and Cash
- Go to "Accounting / Reporting / Audit Reports / Trial Balance"
- Download "COA SAT (XML)"
- Validate the XML on an online SAT document validator (e.g. https://ceportalvalidacionprod.clouda.sat.gob.mx)

**Issue:**
The validation fails because the XML contains lines with incorrect or missing value for "CodAgrup" attribute.

**Cause:**
The "CodAgrup" in the "COA SAT (XML)" refers to the code of the account groups.
The accepted values are defined in the "Catálogo de Códigos Agrupadores" XSD file.
https://github.com/odoo/enterprise/blob/ecce698637dc2ef13dfdb27dfde303a7f1191aaf/l10n_mx_xml_polizas/data/xsd/1.3/CatalogosParaEsqContE.xsd#L4-L1086
The created account [123456789] is put in the root account group with code "1" and a line is added in the "COA SAT (XML)" with this value.
However, it is not an accepted value.

**Solution:**
From the account groups created automatically by MX localization, only the root account groups (i.e. with code "1", "2", "3",...) do not have a valid code for the "COA SAT (XML)".
These ones can be ignored.
This solution is not perfect as it is still possible to create an account group with an invalid code that is not a root account group.
However, handling this use case would require to check that each code is included in the set of valid codes (there is more than a thousand).

opw-4209089

Forward-Port-Of: odoo/enterprise#73647