[FIX] product: correctly consider context key for display_name

Regarding of the `display_default_code` context value, `display_name` is
supposed to return the product code or not, eg:
> product.display_name
> '[FURN_6666] Acoustic Bloc Screens'
> product.with_context(display_default_code=False).display_name
> 'Acoustic Bloc Screens'

But since the context was not considered when accessing this field, it would
always return the cached value, which was set from the first time that field
was read, with the `display_default_code` value used at that time.

tl;dr: `display_name` was ignoring the context once cached.

One of the critical issue was that internal code were displayed on the eshop
cart (not the eshop itself), see `name_short`.

task-2517830

closes odoo/odoo#70671

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>