Update SeedHasMisconfiguredEntropy to return a struct.

The returned struct wraps the status quo return value—whether the seed
has misconfigured entropy—and an optional bool that indicates whether
the seed has any studies that apply to the client's platform, channel,
and version and that are constrained to a limited-entropy-mode layer.

This information is needed in SetUpFieldTrials() to determine whether to
transmit the client's LES value with Variations IDs. A follow-up change
will update the LES-transmitting logic.

Bug: 410008879
Change-Id: I5e90252c930e4229181cbca8222fe4f717ab3db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6846025
Auto-Submit: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/main@{#1501535}