[18.0][FIX] product_variant_sale_price: fix double UoM calculations

[LuvForAirplanes]

Fix issue where changing the quantity on a sales order line after the UoM had been changed resulted in a double UoM conversion. To re-produce:

1. Create a new sales order and add a line item with, for example, Gal as the UoM
2. Change the UoM to Quart
3. Attempt to update the quantity. The price will be double UoM converted, thus causing the it to drop drastically from the correct price.

The last time this code was touched was 9 years ago. I believe Odoo since added this as a standard feature.

[LuvForAirplanes]

Actually, I believe this was attempted several times before:
#343 #259 #54

[LuvForAirplanes]

@pedrobaeza Can we get this merged?

[pedrobaeza]

This requires reviews: https://github.com/OCA/odoo-community.org/blob/master/website/Contribution/CONTRIBUTING.rst#review

[LuvForAirplanes]

@OCA/core-maintainers I am looking for a review.

[LuvForAirplanes]

@pedrobaeza how do I get reviews?

[pedrobaeza]

You can ask PSC or module maintainer, and do reviews of other PRs to get traction.