title: avoid auto-killing activity in android
body: <p>I make an android application for an exhibition (device is on about 8 hours a day). 
  I have an issue: the application quits after about between 30mn and one hour.
  I force the screen not to go into sleep mode, and it works fine. 
  But how could avoid the auto-killing app by android ? Is this beacause of the memory manager ? </p>