[multi-part] Eliminate 1k selection limit

* Save selection to disk, delete old selections on app startup.
* Do clipper work in background for copy/cut.
* Pull DragShadowBuilder out of DirFragment.
* Reorganize selection handling to avoid creation of large lists of objects.
* Eliminate getAll from selection, make it iterable, remove unused method.
* Avoid creating DocumentInfo instances for full selection when initiating drag and drop.

Next step will be to pass selection tokens to FileOperationService.

Change-Id: Ia3e30a88ebf328e5b045d06b0df15dbd26a01a42