Fetch performance class on ModelBroker::Subscribe

This ensure performance class is available when clients call Subscribe()
on the model broker. This matches the behavior of
OptimizationGuideKeyedService::GetOnDeviceModelEligibilityAsync().

This required moving some of the performance class logic from the
keyed service into OnDeviceModelServiceController.

Bug: 415808003
Change-Id: I49d492c61fcc0460bb578c4512a32872562b72ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6519654
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457167}