This was turned on implicitly in libsignal v0.65.3, but was discovered
to have issues. Those issues should now be fixed, but this time we
have a RemoteConfig kill switch in case there are more. That kill
switch also needs to control the use of system proxies (cf. 1f6fea1a).
Since this already affects more than just the chat connection, having
it in OWSChatConnection isn't really the best choice. This way we can
be sure the libsignal Net instance's proxy settings will be updated
before any notifications go out.
