[16.0][FIX] hr_expense_invoice: Incorrect data and check when creating vendor bill from expense

[pedrobaeza]

Two fixes:

1. The "amount_total" field now reflects the total of the invoice including taxes, so we have to use "untaxed_amount".
2. If changing any data in the linked vendor bill, the computed fields used for the comparison are not yet recomputed, so let's do the check on a later stage (after the write).

Fixes #245

@Tecnativa