state-routes: only save route properties if it is available

This MR improves the logic that saves the routes properties by only saving them if the route is available.

Some ALSA devices seem to reset the properties of a route when the route becomes unavailable (Eg when unplugging the headset from the Jack port). This fix avoids saving the newly reset properties into the default-routes state file when that happens. This makes sure the old properties are always restored when the route becomes available again (Eg when plugging the headset back into the Jack port).