fix(focus indicators): Improve styles that determine when focus indicators should render on selection controls.

[zelliott]

For accessibility reasons, it's usually a bad idea to nest interactive elements such as buttons, additional selection controls, form fields, etc inside of Material's selection controls
(i.e. checkbox, radio, slide toggle). In practice, it's done quite often because it's an easy way to visually align content with the selection controls' labels.

Our previous styles were set up such that if the selection control was focused, all of the focus indicators nested within that selection control would render. Instead, only the focus indicator associated with the particular selection control should be rendered. These new styles do that.

[crisbeto]

LGTM

[jelbourn]

LGTM

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