[FIX] account_anglo_saxon: do not put stock accounts on services

Services are not moved in stock so invoices created from stock.picking should
not use stock accounts, same when creating lines for sales. opw 615263
[FIX] account_anglo_saxon: keep reference to stock.move on invoice line

When an invoice is created from a picking keep a reference to the stock.move to
use the right price for the COGS entry.
This used to be done by the _prepare_invoice_line method but it was removed
during WMS refactoring, replacing by the brand new _get_invoice_line_vals
method! opw 615263