[FIX] product_expiry: send alert activity to the correct user

Steps to reproduce:
1. Have 2 companies, and select the one w/ the highest ID
2. Create a new product tracked by lot + expiration date
3. Receive product
4. Set the received lot's expiration and alert dates in the past
5. Inventory > Operations > Run scheduler

Before this commit:
An activity is created on the lot for OdooBot,
despite the product's responsible set to the current user.
It happens because the default value for `responsible_id` is the current
user, but the value is only set for the current company. When the
scheduler runs, it doesn't set the company; therefore, it takes the
product's responsible user for the company with the lowest ID, which is
not set.

After this commit:
The activity is created for the product's responsible user using
the lot's company.

opw-3489340

closes odoo/odoo#135361

Signed-off-by: Tiffany Chang (tic) <tic@odoo.com>