Refactor code

* Rename ScrollDirectionDetector -> AbsListViewScrollDirectionDetector
* Rename ScrollDirectionRecyclerViewDetector -> RecyclerViewScrollDirectionDetector
* Make AbsListViewScrollDirectionDetector, RecyclerViewScrollDirectionDetector and ScrollDirectionListener package private
* Convert class members in FloatingActionButton class to local variables(mOnScrollListener, mRecyclerViewOnScrollListener, mListView and mRecyclerView)
* Set scroll threshold with a setter