crostini export with notifications

Exposes export and import functions which control
File dialog chooser to select file for export/import
and displays notification with progress.

BUG=912638

Change-Id: I7b5ff8151bf76f81d2483ff199ed573e42277a2e
Reviewed-on: https://chromium-review.googlesource.com/c/1462638
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632495}
18 files changed