Reserve the setOnScrollListener() method for PagingListView.

Reason:
When using the PagingListView, we usually use the OnScrollListener to get the scroll event for UI animation, so it is necessary to reserve the setOnScrollListener() method. 

What I did:
1. Override the setOnScrollListener() and keep a local listener instance.
2. In init(), invoke super.setOnScrollListener() instead, and dispatch event to local listener.