Cleanup of Selection APIs.

Improved consistency in naming, updated some docs...
    baby steps, much more documentation is needed.
    Replace "Manager" noun w/ "Helper" noun for better
    consistency with RecycleView naming standards.
Cleanup idiosyncracies in usage is DocumentsUI.
Make SelectionManager responsible for notifying RecyclerView of selection changes
    allowing all notifications to be isolated in the listener interface.
    This also eliminates a requirement that Adapter implementations add
    plumbing to dispatch item change notifications to RecyclerView.

Bug: 64847011
Test: Increased coverage.
Change-Id: Icdf3815f2f4bbce8751d740d362228a204666f35