If it's only a change in the desktop app, wondering if it would be a separate setting on the desktop app settings page instead? Just because the "idle" state wouldn't mean anything for those that don't use the app.
One thing I'm a bit uncertain is how this would work if someone had the Mattermost instance open on the browser and the desktop app.
If they're "away" from Mattermost for 10 minutes, but still at their computer, then they'd be "away" on browser but "online" on desktop app. I suppose the "online" state would be shown to other users?