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OS Settings: Update to use preprocess_grit

OS settings is the last remaining user of unpack_pak. By migrating it to
use preprocess_grit, unpack_pak can be deleted, preventing any new UIs
from adding more usage. All UIs should instead use preprocess_grit
(and, eventually, autogenerate grd files with generate_grd).

Planning to update OS settings to autogenerate grd files in a followup.

Bug: 1132403
Change-Id: I5378836543befb224a82e58b0211bafba9aead64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487885Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819485}