WebRTC privacy-hardening settings

As noted in mullvad-browser#151 (closed), the most sensible preference changes to harden WebRTC privacy for our use cases seem to be:

• media.peerconnection.ice.relay_only -> false
• media.peerconnection.ice.default_address_only -> true
• media.peerconnection.ice.proxy_only_if_behind_proxy -> true