Crostini: re-share all paths with container at startup

Read list of persisted shared paths from prefs, and re-share
with seneschal/9p when container starts.

Bug: 878324
Change-Id: Iea0ff71b79e5787d06b9d1b42a985cea2fb2631b
Reviewed-on: https://chromium-review.googlesource.com/1232793
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592724}
7 files changed