[FIX] point_of_sale: consider scanned price as manual price

In a POS session, when using the scanner, if the seller changes the
customer, the prices may become incorrect

To reproduce the issue
1. Create a product P:
    - Sales Price: 38
    - Barcode: 2312345000002
    - Available in POS: True
2. Start a POS session (with debug Window)
3. Scan 2312345010001
    - This is product P with price $10
4. Set a Customer

Error: The price is now $38

Because the price has not been set manually, when changing the customer,
the pricelist is updated and so does the price.

When scanning a barcode that includes a price, the latter should be
considered as manually set.

OPW-2618934

closes odoo/odoo#76063

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>