fixed issues #910, #909 and #906

Feature_Backpack_Enabled can be set in build.gradle

changed the strings accordingly

removed checkstyle warning

added check if BuildConfig.FEATURE_BACKPACK_ENABLED is true

fixed TODO
add tasks to switch to old branches
Merge pull request #915 from Catrobat/issues910_909_906

fixes [CAT-917]
fixed issues #910, #909 and #906
Merge pull request #1029 from Catrobat/switchToOldBranches

add tasks to switch to old branches
added buildAPK.gradle with all apk generator related tasks