Don't proxy calls to DocumentsAdapter in MultiSelManager.

We were bouncing calls out to the adapter pulled from RecyclerView,
but the adapter is easy to write a test double for and
we can readily inject the adapter when needed at runtime.

Eliminates unnecessary indirection.
Also, protect against failure when documents can't be loaded for delete.

Change-Id: Ief6585bf2e3e4fd407d801d485a9d7cd888b8500