Enhance DragAndDrop ListView scroll performance

This change is not needed for API v14 or above (not tested on Honeycomb).
Verified by logcat GC output.

See http://stackoverflow.com/a/10673046 for more details.

[Successful testrun](http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/315/)