Support for long click and pressed state

Based on code from AbsListView, added support for long press so that an
onItemLongClickListener will be called for a long press on cards, and
added the pressed state for cards when they are clicked or long-clicked.
Added checks for motionPosition >= 0

Added checks to verify that motionPosition is not negative
Added long press handler to sample

Added onItemLongClickListener with a toast to the StaggeredGridActivity
sample.
fix merge conflict
Add updateEmptyStatus() in setAdapter() method