authpolicy: Move preg_policy_writer to separate file

Preparation for preg_policy_encoder unit test, where the writer will
be used as well.

BUG=chromium:664440
TEST=Compiled, ran tests.

Change-Id: Idd6ccc064d4fcae7c6c5a1096b4e93259d03b714
Reviewed-on: https://chromium-review.googlesource.com/469611
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@google.com>
5 files changed