[REF] phone_validation: class attribute to bypass the length validation

before this commit, if user need to bypass the validation
added for the length in custom module, the entire function
has be rewritten in the custom module.

scenario:

* add the phone_mobile_search field to the name search of res.partner model

* then open sale order form, and in the customer field enter any letter or digits, this user
error will be raised

after this commit, users just need to change the class
attribute: _phone_search_min_length in the
inherited module.

closes odoo/odoo#122248

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>