redid issue #30 fix
Modify Keep Screen On code in Stage

* Use addFlags() instead of setFlags()
* Remove hardcoded int in tests

Tested on device with 15s screen timeout. Works.
Fix SimpleStageTest

* Remove goBackToHome() because nothing exists besides the
  StageActivity
* Remove assert that was just there to trick the SourceTest
Merge pull request #151 from Catrobat/DavidR

WakeLock for Stage implemented