bug(material/checkbox): VoiceOver interprets the SVG inside a checkbox as an "empty unlabelled image"

[zelliott]

Reproduction

Steps to reproduce:

1. Go to https://material.angular.io/components/checkbox/overview on Chrome
2. Turn on VO.
3. Linearly navigate to and past the checkbox via VO + arrow keys.
4. At some point, you should hear "To get missing image descriptions etc etc... empty unlabelled image". The virtual cursor is on the SVG element.

Expected Behavior

The SVG element should be hidden from screen readers. The fix is likely as easy as adding aria-hidden="true" to the root SVG element.

Actual Behavior

The SVG element was accessible to screen readers and unlabelled.

Environment

• Angular: Latest
• CDK/Material: Latest
• Browser(s): Chrome (maybe all)
• Operating System (e.g. Windows, macOS, Ubuntu): Presumably all

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