mtpd: fix seccomp policy

After the refactor it looks like the message loop uses some
different syscalls, and we have a few extra syscalls to get the
uid/gid for the process as well. Prevent seccomp from killing
mtpd when it uses these new syscalls.

BUG=chromium:761247
TEST=deploy to minnie, cyan and successfully connect an
  MTP-supporting android device

Change-Id: I9cef6ec24b2dcae0f3b4e56742688d7297f1a928
Reviewed-on: https://chromium-review.googlesource.com/648077
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
3 files changed