added OnStickyHeaderChangedListener

I added a `OnStickyHeaderChangedListener` to support a UI that reacts to the current sticky header.

E.g. imagine the headers reflect a date, and when you scroll to a new date, elements elsewhere in the UI update using that new date.  Without this `OnStickyHeaderChangedListener`, the UI would have to attach it's own `OnScrollListener` and duplicate the headerId change logic from `StickyListHeadersListView#updateHeader`.