[FIX] point_of_sale: adapt _create_account_move_line for inheritance

This commit intend to allow better inheritance in the
account_move_line_creation_process

The new _prepare_account_move_and_lines method only prepare
the move and the lines but does not write and post them.

This is done by the former _create_account_move_line method.

Needed to fix pos_sessions with unbalanced journal entries
caused by round globally option with taxes included in price

It is a backport of https://github.com/odoo/odoo/commit/52d68984459210cc1a1da039707eb47ae937da07



closes odoo/odoo#39108

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>