res_pjsip_mwi: update unsolicited MWI subscriptions on updating contact

Do not need to unsubscribe/subscribe on creating the ednpoint's contact.
The modified function create_mwi_subscriptions_for_endpoint adds
the subscription only if it does not exist.

The subscriptions aren't added for active contacts
which are retrieved on startup from realtime
if mwi_disable_initial_unsolicited=yes.
Because the mwi_contact_added is not called.
So the subscriptions also should be created on updating contact.

ASTERISK-26230 #close

Change-Id: I47e265af9296ca09aa42a316fdacac104148cee4