fix(material/autocomplete): options inside groups not read out with VoiceOver

[crisbeto]

We're currently hitting a bug with mat-autocomplete that prevents options inside of mat-optgroup from being read out by VoiceOver on Safari. These changes implement an alternate accessibility pattern that only applies to options inside an autocomplete since the work correctly in mat-select.

[jelbourn]

I got confirmation from the original reporter of the issue that, as your observed, a recent update to either Chrome or macOS fixed the underlying issue for Chrome + VoiceOver. So, now the question becomes: do we break normal option group reading for Chrome to get options to do a workaround for Safari? I'm leaning slightly towards just leaving it as is and considering it a Safari issue.

[crisbeto]

Without any data whatsoever, I was under the impression that most VoiceOver users are using it together with Safari which is still going to be broken. What if we limited the behavior from this PR to Safari with a TODO to eventually remove it, if it gets fixed in VoiceOver?

[jelbourn]

Yeah, that seems reasonable, however unpalatable browser detection may be.

[crisbeto]

Changed to only use the workaround on Safari.

[jelbourn]

LGTM

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