[FIX] point_of_sale: POS user access rights

 - A simple POS user doesn't have the rights to write
   on pos.config, not even to write the `current_session_id`,
   so it must be done as sudo.
 - If no fields is specified to `search_read`, it reads all fields.
   The simple POS User cannot access all fields on `account.journal`,
   (e.g., the computed fields implying `account.payment` records)

opw-659079