SearchPrefetchService supports Incognito profile

Search prefetch feature is currently available only on non-Incognito
profiles without obvious reasons.

This CL changes SearchPrefetchServiceFactory to return the instance even
for Incognito profile, not only for regular profiles, which allows the
browser to trigger search prefetch in omnibox.

The actual prefetch triggers are managed by existing feature params.
There is no change.

This is implemented behind the feature param named `allow_incognito`.

Here is the design doc (internal):
https://docs.google.com/document/d/1yP49fFU4VIlrIBOgDSqYqqpnH46Xx2iv_Ckh9censfs/edit?resourcekey=0-DeNv8KgxA-sl-VpLzbw10w&tab=t.0

Change-Id: Ife4ae3e937395e253ae1ce574cf743c1076cfbd5
Bug: 394716358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6186527
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417988}