Add crostini unshare path

Unshares a path previously shared, and removes path from prefs.

Bug: 878324
Change-Id: I9655c16851eed2eccca225fc3d9642db21b51dde
Reviewed-on: https://chromium-review.googlesource.com/c/1317212
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608290}
6 files changed