Reland "shill: start running shill process tree in a minijail"

This is a reland of 8751e2a8474c895d11d56ca45e4f71e808b25c27.

The only reason we reverted the change was to merge the revert back into
M69, but we still want the changes in TOT/M70 -- hence the reland.

CQ-DEPEND=CL:1162312
BUG=chromium:649417
TEST=By all accounts networking on M70 was working before we reverted
yesterday. This just brings us back to that state.

Change-Id: Ia1b7f38384d53fa4ed4806b7248fdacbfd1bf7b8
Reviewed-on: https://chromium-review.googlesource.com/1162311
Commit-Ready: Micah Morton <mortonm@chromium.org>
Tested-by: Micah Morton <mortonm@chromium.org>
Reviewed-by: Micah Morton <mortonm@chromium.org>
Reviewed-by: Kirtika Ruchandani <kirtika@chromium.org>