fix(material/slide-toggle): m3 slide-toggle track

[wagnermaciel]

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[wagnermaciel]

@crisbeto @mmalerba

I don't love naming the tokens visible- and hidden- but this was the best I could come up with

I left comments explaining what is meant by "visible" and "hidden" in 3 files. I wasn't sure which file to leave the comment in since all 3 are technically defining the variables

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