Remove image_decoder dependency from c/b/ui/ash/thumbnail_loader

This CL removes the dependency on `chrome/browser/image_decoder` from
the `thumbnail_loader` component. This is part of a larger effort to
eliminate direct dependencies on `//chrome` from Ash Chrome.

The code will be updated to use `data_decoder::DecodeImageIsolated`
directly, which is the underlying API that `ImageDecoder` was wrapping.

This CL (and the commit message above) was generated by Gemini CLI with
a few human intervention.

Bug: b:428654878
Change-Id: Ib4fab33f6236cd8d1c08806a6a4d7cf9a575a4d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6799360
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Jun Ishiguro <junis@google.com>
Cr-Commit-Position: refs/heads/main@{#1496691}