terminal: Simplify LoadIconsFromResources code

Replace the callback-driven LoadIconsFromResources with FromVectorIcon.

Also, rebuild the vector_icon to use
ui/file_manager/file_manager/foreground/images/volumes/linux_files.svg
instead of chrome/app/theme/crostini/crostini_mascot_1x.svg (since
deleted).

This slightly changes the rendering of the new tab menu:

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=538562&signed_aid=t-7gbXeondkvFNWl3WIUvw==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=539202&signed_aid=SdGKXBsBfmdJRra87vmUBA==&inline=1

The right-click menu doesn't change. Also, the shortcut icons are meant
to be larger than the rest of the icons in the right-click menu, so we
keep the same size as before: kAppShortcutIconSizeDip.

Bug: 1241262
Change-Id: I5f41fde3025d3275f4020b88bf2979a5c04f4f08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3414175
Reviewed-by: Jason Lin <lxj@google.com>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Emil Mikulic <easy@google.com>
Cr-Commit-Position: refs/heads/main@{#963848}