fix some lint warnings

see commit message for additional information.

lint warnings current master branch: http://jenkins.catrob.at/view/Util/job/Catroid-Code-Checker-master/1/androidLintResult/?
lint warnings this branch: http://jenkins.catrob.at/view/Util/job/Catroid-Code-Checker/41/androidLintResult/?

multijob run: http://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/99/
emulator tests of failing testcases: http://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-emulator/85/
http://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-emulator/86/