fix(radio-button): when no value selected, first button not announced on focus

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

• Screen readers don't work properly (in different ways, see issues) with md-radion-group and md-radio-button.
• The radio button demos are not accessible.
• There is a lot of extra stuff in the radio button demos that makes them more complex than needed, including unused styles, extra elements and attributes, etc.

Issue Number:
Fixes #11973. Fixes #11923. Fixes #11974.

What is the new behavior?

Screen reader behavior is improved/fixed.

• when no value selected, set aria-activedescendant to first button's id
• properly define labels and aria-labelledby in demos
• improve some JSDoc and types
• rename some variables to make them more readable
• remove unused parameters
• improvements to multi-column demo
  • remove unneeded or unused styles and elements
  • use aria-describedby to make first column available to screen reader users

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mmalerba]

LGTM