[login_manager] Share 'is developer system' logic.
[login_manager] Share 'is developer system' logic.

xstart.sh and init/hotkey-access.conf were duplicating logic.
That logic has been extracted to a tool in /sbin now.  Use it.

BUG=None
TEST=install test image on a device; only 2 VTs should be available.
TEST=install base image on a device; no VTs other than the UI should be reachable.

CQ-DEPEND=Ief1aa40a187ecac34bb222cac4b6d4ebc4880ee1

Change-Id: I5c073f1e2dcda97d590d5e1952dd1d09bf0730e0
Reviewed-on: https://gerrit.chromium.org/gerrit/38245
Tested-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Reviewed-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Commit-Ready: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager