Spoof Network ID

Mozilla 1561005 added a unique string identifier to nsINetworkLinkService.idl. It's at least partially based off of a hash of the user's IP and MAC address. It's not clear to me what it's used for, but we should probably stub out the implementations.

Relevant ticket:

https://bugzilla.mozilla.org/show_bug.cgi?id=1561005

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