Split out helper functions from GetFilteredJSONPolicies

GetFilteredJSONPolicies is a bit too long.
Split out a separate helper function to keep things short.

Bug: b:254500509
Test: ./out/Default/bin/run_unit_tests
--gtest_filter='ArcPolicyBridgeTest*'

Change-Id: I5e6f86baa10390e458eb9714b994615750f29004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3970160


Commit-Queue: Yao Li <yaohuali@google.com>
Auto-Submit: Yao Li <yaohuali@google.com>
Reviewed-by: Muhammad Hasan Khan <mhasank@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1062381}