res_pjsip_mwi: Set up unsolicited MWI upon registration.

res_pjsip_mwi: Set up unsolicited MWI upon registration.

The res_pjsip_mwi previously required a reload to set up the proper
subscriptions to allow unsolicited MWI to work. This change
makes it so the act of registering will also cause this to occur.
This is particularly useful if realtime is involved as no reload
needs to occur within Asterisk to cause the MWI information
to get sent.

ASTERISK-25180 #close

Change-Id: Id847b47de4b8b3ab8858455ccc2f07b0f915f252