AbsListView: add option to disable scrolling cache (1/2)

scrolling cache (discard?) can cause performance lag due to GC
allow user to enable/disable/force scrolling cache on the fly
inspired by AndroidON, http://forum.xda-developers.com/showthread.php?t=1411317

Change-Id: I89605b42182edeafbc4c07e2b7650a914f4f7c9a