add a background watchdog thread which will automatically exit the app if it has been in the background
for more than 2 minutes.. on some Android devices, after n minutes of being paused, with the display turned off,
the resume does not properly restore the screen. instead we get a black screen.
this background watchdog thread will basically sidestep that issue -- since the app will automatically exit
if it's paused for more than 2 minutes, there's little/no chance of getting the black screen on resume..