supl.nokia.com is hardcoded, yes, and used when you don't supply your own supl server. this is not the problem you are encountering, since you do supply a supl server (supl.google.com) when running the proxy.

the supl domain name you created should be used in 3 places: (1) in the domain of the certificate you created, (2) in Settings > Location, and (3) in /etc/hosts. Can you confirm this is true in your case?