extensions: Fix failing unit test on android

Fix the test started failing after https://crrev.com/c/6370848.

The test PolicyCheckTest.PolicyFailure was just accidentally passing
despite PolicyCheck::Start uses management_policy() which is always
nullptr on android. The change not to access the policy made the test
fail, so we ifdef-out the failing expectation on android, adding a TODO
to revisit it.

Bug: 392777363
Cq-Include-Trybots: luci.chromium.try:android-desktop-15-x64-rel
Change-Id: Ib656af62072974e7c527cdb13197e84573c5e476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6390135
Owners-Override: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Keigo Oka <oka@chromium.org>
Auto-Submit: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437313}