[Files SWA] Filter out resources for untrusted contexts

Resources under u/f/f/foreground/elements/sandboxed/ are intended to
handle untrusted contents and to be exclusively loaded in untrusted
contexts (webview or chrome-untrusted://file-manager/).

Currently, all the file manager resources are added under the Files SWA
WebUIDataSource, which makes it possible to load the resources in
u/f/f/f/e/sandboxed/ under chrome://file-manager/. This changer filters
out these untrusted resources.

The logic to filter these out is resources is moved to a new
resource_loader.h/.cc file.

Test: ash_webui_unittests --gtest_filter=ResourceLoaderTest.*
Bug: b:197601728
Change-Id: I3265dd237a3dcd1de0f38120d2eb61f3042c91cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3181780
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Bo Majewski <majewski@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#925641}