[FIX] core: order of fields in onchange of SSF

When calling the initial (create / default_get) onchange, the SSF
would send the list of fields in whatever order was provided by the
fields map of fields_view_get.

The web client uses view ordering, and it turns out some uses / tests
have dependencies between onchanges (e.g. _create_payment in
test_account_reports) which break on some orderings of the fields.

Send the initial onchange using view-ordered fields in the SSF as
well.

closes odoo/odoo#31494