Simplify CopyService in DocumentsUI.

This CL simplified the logic by:
* Passing always DocumentInfo, instead Uri/DocumentInfo depending on method.
* Remove redundant logic by calling copy() recursively.
* Which also fixes efficient copy if only some of the files can be copied
  efficiently.

Bug: 26176231
Change-Id: I4444077194d0bae3c149c9a012c325e068f946ed