feat(expressive): More expressive updates for Display size & text

- Updates each preview page to include a unique title
- Updated preview padding to closer match UX mocks and to look more
  consistent across the 3 default previews.
- Uses categories to group the sliders separate from the toggles.
  Includes titles for the categories so that these categories are
  understandable for screen reader users.
- Updates the Message preview icons to use a black icon color
  (the old gray color had a very low contrast against the icon bg)

Fix: 399173131
Test: atest com.android.settings.accessibility
Test: see screenshots in bug
Test: interacted with the previews using TalkBack; observed that
      TalkBack still reads each preview's content description
Flag: EXEMPT low risk visual changes
Change-Id: Ied418adc66931b6665de9f1454d7ac1bdb4b0e87