[Compose] [Segmentation] Create a default heuristic model.

This is hooked up to the existing nudge probability param. For that
reason, the segmentation feature is switched to default-enabled, in
order to keep the existing behavior consistent.

Bug: b/337951526
Change-Id: Ia909dc61ee76e670089c5984cb92d81ad7444124
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5531875


Auto-Submit: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300870}