Create Crostini Feature and Flag for Crostini USB support

Adds feature for Mounting Usbs into Crostini.

Bug: 891381, 899568
Change-Id: I36f5cdf88d2901b24c9ef83671418fae64f275ad
Reviewed-on: https://chromium-review.googlesource.com/c/1301095
Commit-Queue: Josh Pratt <jopra@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603748}