Added visibilityTrackerListener

VisibilityTracker.java

    public ImpressionAdapter(Activity activity, List<String> dataSet) {
        mDataSet = dataSet;
        mVisibilityTracker = new VisibilityTracker(activity);

        mVisibilityTracker.setVisibilityTrackerListener(new VisibilityTracker.VisibilityTrackerListener() {
            @Override
            public void onVisibilityChanged(List<View> visibleViews, List<View> invisibleViews) {
                handleVisibleViews(visibleViews);
            }
        });
    }

    private void handleVisibleViews(List<View> visibleViews) {
        Log.d(ImpressionAdapter.class.getSimpleName(), "Currently visible views \n");
        for (View v : visibleViews) {
            Integer viewPosition = mViewPositionMap.get(v);
            String viewTitle = mDataSet.get(viewPosition);
            Log.d(ImpressionAdapter.class.getSimpleName(), viewTitle);
        }

        Log.d(ImpressionAdapter.class.getSimpleName(), "------------------------------");
    }