WebUI: Remove some unnecessary Polymer dependencies in cr_elements

- Change cr-toolbar-selection-overlay to use CrA11yAnnouncer instead of
  IronA11yAnnouncer
- Remove imports of paper-styles/color.js from places where no paper-
  colors are actually used.

Bug: 40943652
Change-Id: I622b8d270d2e6f9dcb1d28003c5d2cd393e60ec8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425423


Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283174}