fix(toggle): on-off icon is not announced by screen readers

[amandaejohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• 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.
• Build (npm run build) was run locally and any changes were pushed
• 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:

• 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?

The icons rendered when enableOnOffLabels is turned on may be read by screen readers, despite being purely decorative.

Issue URL: N/A

What is the new behavior?

Icon now has aria-hidden="true".

Does this introduce a breaking change?

• Yes
• No

Other information

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