Fix focus highlight on PWM settings page.

<cr-link-row> uses `:last-of-type` to trigger rounding its bottom
corners so a hidden <cr-link-row> at the end will break that. Thus the
passkey management row needs to be added & removed from the DOM using a
`dom-if` instead.

Fixed: 1433617
Change-Id: I0f99097e40c22da493b15c6e4dda31e663b62ce4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4435749
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Commit-Queue: Viktor Semeniuk <vsemeniuk@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1131811}