Transition selection to use Model IDs.

This CL transitions the MultiSelectManager to (mostly) use Model IDs.

- Add the ability to retrieve all model IDs for the current directory,
  from the model.

- Add a map in the DocumentsAdapter that maps from adapter position to
  model ID.

- Make the adapter listen for model updates, and update its internal map
  of positions to model IDs appropriately.

- Use the aforementioned map when binding ViewHolders.

- Get unit tests to compile; get as many tests passing as possible at
  this point.  Tests related to deleting things won't work right now.

Still to do:

- Add code to the adapter to sort and group items.  After this is done,
  SortingCursorWrapper will no longer be needed.

- Add code to the adapter to deal with item addition/removal.  After
  this is done, the pending-deletion code in the model can be removed.

- Rationalize position-based vs model-based selection.  Some code in the
  MSM (in particular, code dealing with range selection) is still
  position-based.  It's becoming clear that it doesn't make sense for
  range selection to be ID-based, since "range" is a concept that only
  makes sense in the context of items that are positioned in the UI.
  Will need to iterate more on the position-based code to make sure it
  makes sense.

BUG=26024369

Change-Id: I767cde2d888c101aaf83b59155b14634a236164b