dPWA: File Handling - add stub settings control

This CL adds plumbing and a toggle in the app management
page on WML. It's only visible when the file handling API
is enabled and the app makes use of the FH API.

This control will become more complex in the future (see
mocks attached to bug), but for now is just a toggle.
This will also be added to the Chrome OS app management
page for PWAs, but for now that's left as a TODO.

Bug: 1245293
Skip-Translation-Screenshots-Check: True
Change-Id: I46414f114e448ac4ed5134a14608131d3206fd8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3459440
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Jeevan Shikaram <jshikaram@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975165}