Refactor mobile proxy code to create a peer connection only after we receive an offer
Right now the peer connection is created on startup. In order to be more flexible with ice servers, and to eventually support more than one client at a time, we should move the creation of the peer connection to the fetchOffer()
loop (this is what the webextension does: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake-webext/-/blob/master/snowflake.js#L58 ) or until after the proxy receives a client offer (which is what the standalone Go application does: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/master/proxy/snowflake.go#L401 ).