Add support for flagging extensions without privacy practices

Add support for displaying extensions in the Extensions Safety
Hub that have not disclosed their privacy practices in their
CWS listings. Note that this CL uses existing strings for off-store
extensions as a placeholder. Once the real strings are finalized, they
will be added in a follow-on CL.

Bug: b/334110393
Change-Id: I2c4d6b2396db2ee46932f18a6a1e59f198566eaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5462027
Commit-Queue: Adam Psarouthakis <psarouthakis@google.com>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Side YILMAZ <sideyilmaz@chromium.org>
Reviewed-by: Anunoy Ghosh <anunoy@chromium.org>
Reviewed-by: Richard Chen <richche@google.com>
Cr-Commit-Position: refs/heads/main@{#1291912}