Add CrostiniExportImportUIAllowed user policy

CrostiniExportImportUIAllowed is a user policy that controls
whether the export / import UI is shown to a user where
they can export and import container images in crostini.

Bug: 912638

Change-Id: Ia28d1d43ca5bfbb047c6f870a1de7b4d46605fe8
Reviewed-on: https://chromium-review.googlesource.com/c/1468223
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632374}
6 files changed