[FIX] mrp: UoM of quantity available

The field `quantity_available` can be computed in two different ways:
- equal to `product_uom_qty`
- equal to `reserved_availability`

However, `product_uom_qty` is in the UoM of the move, while
`reserved_availability` is in the UoM of the product.

We convert into the UoM of the move, because `quantity_available` is
supposed to be in the same UoM than `product_uom_qty` ("To Consume").

opw-704775