fixed issue #797: handle audio focus in StageActivity
Merge pull request #975 from Catrobat/issue#797

fixed issue #797: handle audio focus in StageActivity
Added exceptions when loading projects has errors
UtilFile.java/loadExistingOrCreateStandardDroneProject updated
DroneBrickLayoutTest.java/testBricksPrototypeView updated
MainMenuActivityTest.java/testLoadProject modified
MainMenuActivityTest.java/createTestProject modified