[IMP] all: read_combined(['arch']['arch'] => get_combined_arch()

The method read_combined() has been deprecated in favor of explicit
calls to read() and get_combined_arch().

There are places where we call _get_combined_arch() instead, in order to
avoid parsing the XML that has just been serialized. This saves useless
serialization-deserialization.

Task: 2541577
Meta task: 2463632

Co-authored-by: Fabien Pinckaers <fp@odoo.com>
Co-authored-by: Raphael Collet <rco@odoo.com>