system_api: Add 'bool for_login_screen' to arc.proto

BUG=b:32308760
TEST=cros_run_unit_tests --board=caroline --packages chromeos-login

Change-Id: Ia0d5cbdec1bcbd055e280ac2d766b0143c4341aa
Reviewed-on: https://chromium-review.googlesource.com/520167
Commit-Ready: Yusuke Sato <yusukes@chromium.org>
Tested-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
1 file changed