Use the copy of brillo-clang-format instead of the symlink

Currently the .clang-format file in update_engine points to
../../build/tools/brillo-clang-format which is a path in AOSP source code
workflow and doesn't exits in CrOS side. This just copies that file here so we
don't have to revert these relevant changes on merges.

BUG=none
TEST=clang-format -i ./*; git clang-format

Change-Id: I66990527576deb7dee3e29606f056102f966e633
Reviewed-on: https://chromium-review.googlesource.com/965264


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>