fix(header): hide from screen readers when collapsed

[amandaejohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• [x] Build (npm run build) was run locally and any changes were pushed
• [x] Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

What is the current behavior?

Collapsed headers are read by screen readers, causing duplicate header announcements when using collapsible large titles.

Issue URL: N/A (internal ticket)

What is the new behavior?

Collapsed headers now have aria-hidden="true".

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Other information