fix unstable test

since the merge of GSoC backpack, testProjectDetailsLastAccess failed from time to time (you can click on "older" to get to the first build where it failed; test never failed before)
https://jenkins.catrob.at/job/DeviceTest/592/testReport/junit/org.catrobat.catroid.uitest.ui.activity/MyProjectsActivityTest/testProjectDetailsLastAccess/history/?

System.gc() call was merged with gsoc_backpack branch and caused several unstable testruns.
single testrun without System.gc() call - https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-device/105/console