goa_identity_manager_get_identity_finish should return a new ref

The vast majority, if not all, of *_finish functions out there transfer
ownership of the return value to the caller. This also matches with the
behaviour of new APIs like g_task_propagate_pointer. Therefore, let's
make this change for the sake of consistency and making it easier to
port to GTask.

https://bugzilla.gnome.org/show_bug.cgi?id=764157