crostini: filesapp: import crostini image files (.tini) from files app

BUG=972848

Change-Id: I34460845917cf1032ddba9198eb0593a68c7a42b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1723931
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Julian Watson <juwa@google.com>
Cr-Commit-Position: refs/heads/master@{#682683}