Better concurrency for FileManagerService name creation

When unique child name constraints are enabled (NXP-22421), it becomes possible for two concurrent document creations (using Drive, parallel file upload, etc.) to provoke a "segment name" collision, if the files have different names but with a long enough prefix so that PathSegmentService returns the same string.

In this case, it would be useful to have an option in the FileManagerService to create the segment name based on a random value or any other value guaranteed to be free of collisions.