brightness: Add auto-brightness toggle to Settings app behind flag

This CL adds an auto-brightness toggle to the Settings display page,
hidden by default unless the flag
"#enable-brightness-control-in-settings" is enabled. The toggle
does not display real data or do anything on interaction -- that will
be added in future CLs.

Screenshot (flag enabled):
https://screenshot.googleplex.com/9hYPQBPAooUDr3n

Note that the UI does not exactly match the UX spec, mainly w.r.t. the
lack of indent (spec https://screenshot.googleplex.com/8zCvxYE75FYbjzk).
We're waiting for additional guidance from design about this, but that
shouldn't block this CL from landing. Additional UI adjustments will
be made before this flag is enabled by default.

Test: browser_tests -f="*DevicePage*"
Bug: b/328287584
Change-Id: I6643baa0e383a5ee466637c98fa52175f90e5277
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5381446


Reviewed-by: Wenyu Zhang <zhangwenyu@google.com>
Commit-Queue: Camden Bickel <cambickel@google.com>
Cr-Commit-Position: refs/heads/main@{#1280490}