Made LocalFile move/delete/restore handle network partitions better

* Previously, the file existence checks would not distinguish an answer
  in the negative from a non-answer. This was a long-standing problem.
  This avoids moving DB entries without moving the files (unless the
  partition happens in the middle of the moves of course).
* Optimized fileExistsBatch() to do concurrent stats if possible.

bug: 40927
bug: 69312
Change-Id: I9132fd5591bb7a3d5852f17514dcf51a3d8b7812
(cherry picked from commit 8dca7488eceb295708555e732b0d719c143efb41)