With simple sensor detection it is more than adequate to use the main UI thread. Starting up a separate thread with sensors would require a handler and thread cleanup. You are adding unnecessary steps.
If you are already pushing your ui you are likely better off moving some of your other bulk off of the main thread. Sensors are just handled really efficiently with Android already, moving it off the thread won't help as much as it could hurt if you aren't careful.
Now performing operations on the data you get from sensors (like location from GPS) is a whole different ballgame, and something that will very possibly need it's own thread.