Changes

filesystem: JsonEditor: use NamedTemporaryFile

A plain NamedTemporaryFile is sufficient here: we only need a single
file, not a directory to hold it.  Using NamedTemporaryFile is simpler
(no subdirectory to manage) and gives us a .json suffix for free,
which editors use for syntax highlighting.

Change-Id: If3b0bd0fcc90732407dbd03b9cc883f7abeb948e