Fix some more lint warnings

see commit message for details!

also suppressed warnings from update to Android sdk 18.

lint warnings current master branch: https://jenkins.catrob.at/view/Util/job/Catroid-Code-Checker-master/18/androidLintResult/?
lint warnings this branch: https://jenkins.catrob.at/view/Util/job/Catroid-Code-Checker/57/androidLintResult/?

multi job run (currently in queue): https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/188/