identity: Keep a reference to self during async operations

If the IdentityService object is freed while it's getting the initial
list of identities shortly after start up (i.e., the service is
exiting shortly after starting), then there's a chance the callback
associated with the identity listing operation could be called after
the service is freed (which could lead to a crash).

This commit addresses that problem by reffing the service object until
the list operation finishes, preventing it from getting freed
prematurely.

Fixes: https://bugzilla.gnome.org/727896