### Problem:
When applying a discount on a POS order line using a pricelist, if the unit price has more than two decimal places and the quantity is greater than one, the order line may be incorrectly flagged as manually set due to a rounding mismatch.

This logic was introduced by the following commit: https://github.com/odoo/odoo/commit/775c5c95cf763c14715fd0e82e88ff841371d212

The incorrect behavior is caused by computing the product display price by rounding the unit price first, then multiplying by the quantity, rather than rounding the total.

- Example:
    * Unit price = 30.47619047619047
    * Expected rounded price for 2 quantity: round(unit price * 2) = 60.95
    * Current rounded price for 2 quantity: round(unit price) * 2 = 60.96

The issue only occurs on V16 and V17

### How to reproduce:
    * Create a tax and check **Included in Price**.
    * Add a product with total price (the tax excluded price will be calculated).
    * Check flexible pricelist, and add one to the POS config in settings.
    * Check product prices settings as Tax-Execluded Price.
    * Open a restaurant pos config.
    * Add two units of the product.
    * Go back to main floor and access the table again.
    * Apply the discount (it will not apply).

opw-4731266

Forward-Port-Of: odoo/odoo#208390