REVISIT: remove some lag launching apps and improve scrolling

CHANGES:
    AbsListView:
        Clear cache when aborting previous fling
    Activity manager service:
        Reduce unnecessary locking in message handler
        Move a few locks relating to idle activities from inside
            a foor loop to outside. would rather hold until it's
            done than get stuck waiting for something else to release

Change-Id: Ie59615bfad90e3d7acd4f95367248868497e172d