Key exchanges in SDES can only be done securely with TLS and the option
is disabled by default if not using TLS as the transport protocol.
This setting allows to override this behaviour if the user desires
it (f.e. if the user considers the network his packets go through to be trusted).
We only have a single source of settings, so we should reflect that by
using a singleton. This also reduces our LoC.
This doesn't impair our ability to run tests because there we run with
GSETTINGS_BACKEND=memory
