[FIX] hr_expense: Wrong tax in multi companies

Steps to reproduce the bug:

- Let's consider two companies C1 and C2 where C1 is the parent of C2
- Let's consider an expense product P with T1 as tax in C1 and T2 as tax in C2
- Let's consider an expense alias A defned for C1 and C2
- Let's consider an employee E in C2 with work email address W
- Send from W to A an email with subject [internal reference of P] Test Expense 1000€

Bug:

An expense is created in C2 with T1 and T2 as supplier taxes instead of only T2

Inspired from function _compute_tax_id defined in model sale.order and purchase.order

opw:2507162

closes odoo/odoo#70370

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>