Bug 41599: Always return an empty string as network ID

Firefox computes an internal network ID used to detect network changes
and act consequently (e.g., to improve WebSocket UX).
However, there are a few ways to get this internal network ID, so we
patch them out, to be sure any new code will not be able to use them and
possibly link users.

We also sent a patch to Mozilla to seed the internal network ID, to
prevent any accidental leak in the future.
Upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=1817756