Device tracker: hide_if_away default to true

Many people allow guests on their wifi network. This will pop up all kind of devices on several device trackers depending on wifi.

Now I do not mind seeing these items when the guests are present, but having the default hide_if_away = false is actually just gowing and growing the list of items in the ui.

I would suggest making the default true. There are (I guess) fewer people interested in devices being ‘away’ than in ‘all devices ever visited’