[16.0][FIX] account_payment_term_extension: Display correct amounts in journal items when using foreign currency

[sergiobstoj]

Detected problem:

When creating an invoice for 900EUR (in a company currency with USD) with a Payment Term with multiple terms, for example 30% now, the rest in 60 days, the journal items that are generated are displaying the amount in EUR but with USD currency.

Specifically, total invoice amount in EUR is 900, and converted to USD is 1376.
First payment should be 30% -> 412,8 USD
Second payment should be balance -> 963,2 USD

Currently, this info is displayed as:

Payment terms are being computed considering EUR currency.
As:

• First payment, 30% -> 270
• Second payment, balance -> 1106

Correct computing should be in company's currency. This PR fixes that:

[aritzolea]

Could you add a unit test that would not pass without this fix? Thanks