[Switch Access] Update preview focus ring style

Per the bug filed by our UX designer, update the preview focus ring to
have a solid inner ring, and a dashed outer ring with 4dip dashes and
2dip gaps.

Since Switch Access is the only application of the DASHED focus ring
style currently, I modified that style directly, rather than creating
another new style.

Bug: 1152610
Change-Id: If61810ed594e7f388a8cbf71aaa41a44c2ed3cb6
AX-Relnotes: Dashed focus ring in Switch Access now has a different style.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622757
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#842610}