As far as I can recall the basic Therm-App app
does have a 'hotter than' function but does not have a hi-low
limit function to lock the palette/background but
ThermApp Plus by Bill Kraus (available on the Play Store) certainly does, along with a whole host of other useful functions like fixed pattern noise removal, frame-averaging noise reduction, and much more. (Last time I checked it worked with everything except the Therm-App Pro).
From the ThermApp Plus Play Store page:
Most users find ThermAppPlus gives better IQ, supports features such as noise reduction, customization of palettes, hot spot detection, locking palettes, support for AVI, MPEG, raw TIFF, PNG, Cronista formats, time lapse - many of which the default app does not.
I've met Bill Kraus - he's a nice guy, really smart too - and he has been developing the app for several years. It started out largely for his own benefit because he wanted to see bats in thermal infrared.