Refactor OptimizationGuideModelProviderObservation to allow dynamic observation.

This change modifies `OptimizationGuideModelProviderObservation` to support observing different optimization targets or resetting the observation. The `OnDeviceModelAdaptationLoader` is updated to use this refactored class, simplifying its observer management.

This CL also splits the observe registration in OptimizationGuideModelProviderObservation from the constructor.

Change-Id: If0625c15bd768639a86c8460ed23c7e5c652ec8a
Bug: 445936181
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6976334
Commit-Queue: Raj T <rajendrant@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519635}