Expose section and position within a section for a given absolute position

[jkreiser]

A use case for exposing these properties is determining the column span of a particular position of a RecyclerView using a GridLayoutManager. Using your own custom GridLayoutManager.SpanSizeLookup to implement your own getSpanSize(int) only provides you with an absolute position, and if you want to determine different span sizes for your non-header/footer items, it helps to know which section a particular absolute position is in, or what position in a section a given absolute position is.