[builtin ai] Wire up GlobalState and ModelBrokerState to provide
solution for classifier model.

Added a new `ModelType` to represent the feature -> model mapping, so
that classifier/other features trigger distinct model downloading.

Because this is a temp workaround:
- on_device_internals page isn't updated to reflect this new model
- minimal tests were added
- no asset manager for classifier

Bug: 481796902
Change-Id: I73f8d76cba3db46395eca81ccf61eef252f861c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7604820
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Jingyun Liu <jingyun@google.com>
Cr-Commit-Position: refs/heads/main@{#1598037}