Make CPA drive segmentation on-demand

This CL
- Moves page load observation and trigger outside of segmentation. It
will be handled by the client.
- CPA controller invokes segmentation on demand and gets the results
as opposed to observing the segmentation backend for model computation.

Change-Id: I37f873a1c07433865e412d97164729d002c4f439
Bug: 1322001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3715254
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1022519}