[Nearby] Strings updates on contact visibility page.

Removes the "Contacts" title and updates strings in the description for
each visibility mode to match the latest specs.

Screenshots:
  https://screenshot.googleplex.com/An6Ta8EDzfS3KZU.png
  https://screenshot.googleplex.com/C5WwqifMeCnLhkH.png
  https://screenshot.googleplex.com/C48HQVfgouJrhHF.png

Strings spec:
  https://docs.google.com/document/d/1s-KjC1mM4ZPYeYAO7oI8lXltS_1FaYYfMMnNDdbfRag

Bug: b:168841167
Change-Id: I5facb117242e22b838266c2fa5e2629b221e6794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511852


Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#822914}