[FIX] account: onchange of product and uom should not use currency rounding

As the price_unit and the uom rounding can be more detailed than the currency/accounting rounding (2 decimals as a standard) to cut the number before subtotal is wrong.

closes odoo/odoo#63817

X-original-commit: 1b4005f5
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>