[timed] Do not disable automatic time management when system time changes

With this change timed will not disable automatic time management via NITZ unless explicitly told so via Maemo::Timed::WallClock::Settings::setTimeManual().

When automatic time update is enabled, and the system time is changed, then timed will change the system time back according to the next NITZ signal it receives. Timed will query ofono for NITZ data when it reboots, too.