[FIX] hr_expense: improve computed field performance

If you have large amount of product.template records (e.g. over 40k),
`hr_expense` module can't be installed as you would get MemoryError. It
would consume all memory while computing `can_be_expensed` field.

For that, using `_auto_init` to make it less of a memory hog.

closes odoo/odoo#109729

Signed-off-by: Habib Ayob (ayh) <ayh@odoo.com>