[OsSettingsSearch] Align icons and center input placeholder text.

* Aligns RHS and LHS icons of search input to match search result rows.
* Changes row's selected property to attribute instead.
* Add optional margin to input placeholder text so it appears centered
  for new settings search.  Using conventional centering of placeholder
  does not work because the search input is a div placed next to the
  magnifying glass icon, causing a center of the text to be less than
  centered.

Screenshots:
https://screenshot.googleplex.com/iSb8VuxzVN7 align icons
https://screenshot.googleplex.com/HureEGYMWFk align icons (narrow)
https://screenshot.googleplex.com/FF2hGVoFxe6 'center' placeholder text

Bug: 1056909
Change-Id: If17be29eea183d3822a37c787a72d3f21e78c267
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134657
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758121}