[FIX] account: Fix taxes computation in multi-currency

Steps to reproduce:

- On a company with multi currency
- Set the company currency rate to 1, and the foreign currency
  to 0.273748
- Create a vendor bill
- Set first the foreign currency, then select a product with a
  tax to 21% and a price unit of 155.32
- Go to 'Journal Items', the 'tax paid' line debit is computed to 119.15
- Reselect the foreign currency on form.
- Now the 'tax paid' line debit is computed to 119.16

The total is also impacted as the tax changed.

Explanation:

Before this commit, the taxes was both computed in foreign currency and company currency. However, when setting a new currency or changing the date, the taxes wasn't recomputed but the new conversion rate was applied.
This commit is fixing the issue by applying the same logic as in 14.0: the taxes are always computed only the foreign currency, then the conversion rate is applied to get the accounting balance.

opw-2569668

closes odoo/odoo#79065

Signed-off-by: William André (wan) <wan@odoo.com>