[Site search policy] Do not validate fields for managed SEs

Search engines created by policy are fully validated by the policy
handler, so there is no need to validate them again when the user
interacts with the read-only fields in the search engine edit dialog.

The validation was causing the "Done" button to be disabled when the
user focus on the keyword field for featured search engines, whose
keyword has form "@kw, kw" (with a space).

Fixed: 348165485
Change-Id: I2e54e6e9367c585fe69b9fd4e09887b6174217f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5640777
Auto-Submit: Fabio Tirelo <ftirelo@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1317999}