Recycler+Selection documentation and API fixups.

This *should* be the last in-situ code review for the RecyclerView selection API.
My main goal with this change:

    Ensure support lib folks are happy with the basic shape of the API.
    If not, no worries, let's reshaping the API here where the code lives
    in DocumetnsUI. That'll help ensure any changes will retain
    the capabilities necessary to accommodate the more nuanced behaviors
    defined in DocumentsUI, and help provide more detailed example usage
    to support-lib folks prior to our pulling the code over to support-lib
    for final review.

Note that there is fairly substantial, though informal, documentation on each
class and its role in SelectionDemoActivity. That's a great place to start for
reviewer.

Once folks are generally happy w/ the shape. I'll pull a copy of the code
over to the support lib package where we can continue the reivew.
I'll polish off documentation (especially adding package docs) at that time.

Demo video at: https://drive.google.com/a/google.com/file/d/0B1OqiAcKh66ZTHJhU0xSa1lobmM/view?usp=sharing

Bug: 64847011
Test: Added new GestureSelectionHelper coverage. All else passing.
Change-Id: I86f268ce92929cfc490d92100b7bcf158cd4a7dd