[FIX] purchase: auto-complete bill with multi-currency

When adding several PO to a bill, if they don't have the same currency,
it will lead to incorrect amounts

To reproduce the error:
1. In Settings, enable "Multi-Currencies"
2. Invoicing > Configuration > Currencies:
    - EUR: Active, Current Rate = 2
    - USD: Active, Current Rate = 1
3. Create a PO:
    - Currency: USD
    - Products:
        - One product, no taxes, unit price 1000
4. Confirm PO
5. Edit PO:
    - Qty Received: 1
6. Repeat 3 -> 5 with EUR instead of USD
7. Open a new Bill
8. Add the first PO to the field "Auto-Complete"
9. Add the second PO to the field "Auto-Complete"

Error: Both invoice lines are now expressed in EUR and both subtotals
are equal to 1000 even though the exchange rate isn't 1

This commit suggests not to change the currency of the account move if
the latter already has some AML. Moreover, the amounts must be converted
if they come from a PO that uses another currency

OPW-2573748

closes odoo/odoo#73247

Signed-off-by: Steve Van Essche <svs-odoo@users.noreply.github.com>