Make an on_device_internals namespace.

This will allow more objects to defined internally to the interface
without naming conflicts with other interfaces, and allows some of
the current names to be simplified.

Change-Id: Iff7abc697129662a3e990336120730f4ad56710c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6477450
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Steven Holte <holte@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450216}