on_device_model: Add new functions for platform models

The platform side model will not use the existing Chrome service to
download the model, we will replace it with the ChromeOS DLC service
and use the config in the DLC package.
To make the interface more clear, we introduce a new function to load
the platform resources.

BUG=b:331050878
TEST=Test with crrev.com/i/7062637

Change-Id: If0582610c4d7982b66b90851b2c09d5a8401ce4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5387481


Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Yi Chou <yich@google.com>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282193}