refactor(cdk/platform): add common utility for resolving focused element

[crisbeto]

Based on a discussion on an earlier PR, these changes move the logic for resolving the activeElement while piercing through the shadow DOM into a common helper. Furthermore, they expand the logic to pierce through multiple layers of shadow DOM.

[devversion]

Looks great to me 👍

Should we add a test with multiple layers of shadow dom somewhere? e.g. in the focus trap?

[crisbeto]

Not sure, my concern is that we use this in several places already so would we have to copy the same tests everywhere?

[devversion]

yeah, that's true. Though we are missing at least one test ensuring multiple layers of shadow dom are handled properly. I don't feel strongly though because we can assume the nested shadow DOM layers to behave similar to the top-level one (which we test in various places as you said).

[crisbeto]

I think that we need more test coverage for cdk/platform in general since there is none at the moment.

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