Move local device_id into public settings section

Local ID is going to be used by local auth, so no reason to keep it
private.

BUG=25766813

Change-Id: I7cfb190c77246a4445f813f16c8af353284cc7e9
Reviewed-on: https://weave-review.googlesource.com/1797
Reviewed-by: Alex Vakulenko <avakulenko@google.com>