res_pjsip: Reduce processing when a Contact is updated.

When a Contact is updated the only material change that qualify
support cares about is the underlying configuration for the AOR.
In this case we will update things with the new AOR information but
otherwise the callback to indicate the Contact has changed can be
ignored.

This is because it is only when a Contact is added or deleted that
material changes occur within the qualify support. An update can't
change the URI since it would result in a new Contact so it can be
ignored.

Change-Id: I2f97ebfa79969a36a97bb7b9afd5b6268cf1a07d