lacros: Restrict downloads to MyFiles>Downloads and subdirectories

Chrome OS hides many paths from the user. Users can only browse to
MyFiles, certain Android paths, and a selected set of mount points.
For security-in-depth, Chrome OS restricts download paths to those
directories and their subdirectories.

Many of the path utilities functions used to check for these
directories have ash dependencies that cannot be included in the
lacros build.

For now, restrict lacros downloads to the default Downloads
directory and its subdirectories. We can revisit this as part of
the Files work planned for early next year.

Bug: 1148848
Test: expanded unit_tests coverage for lacros

Change-Id: Ia7c41350dcfc29299579f9e7ecd4078e1eb00505
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2541124
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828011}