[FIX] mrp: set default product on unbuild order

Before this commit, Creating Unbuild order using
 Unbuild Button was resetting Bom as it was calling
`_onchange_product_id` after setting `mo_id` which was
resetting Bom on Unbuild order.

To Fix Issue, We setting `default_product_id` so It would not
reset `bom_id` when `_onchange_product_id` is called.

Fixes: #70216
Odoo Ticket: 2521788

closes odoo/odoo#70218

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>