[FIX] hr_expense: Add inverse on total_amount to update unit_amount

The customer was having an issue where they would create an expense from an email with the wrong cost on the expense.
Then when they would try to change the price on the expense record it would seem find all the way up until the journal was posted.

Issue:
When creating an expense from an email the unit_amount is updated instead of the total_amount.
This would cause issues if you had an attachment because when you would attempt to change the amount through the front-end this unit_amount would never get updated because it was set and there was an attachment (line 265).
However, when you would create a journal entry from this expense, because it has the unit_amount != 0 it would provide the unit_amount instead of the total_amount.
Thus propogating the original number from the email even if it was updated between the time of posting and the creation of the expense from the email.

Solution:
Implement an inverse function on total_amount that will update the unit_amount to total_amount_company.
This will cause the unit_amount to stay up to date with the total_amount while also taking into account currency.

opw-3286372

closes odoo/odoo#122012

Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>