fix(a11y): export ActiveDescendantKeyManager

[epelc]

Follow up from #4285 (comment) for exporting ActiveDescendantKeyManager.

Still not sure on guidelines but it seems to be approved and this is very useful for custom overlays(easy a11y ftw).

[jelbourn]

I don't think that the key managers are quite ready for public consumption yet; we plan on adding them to @angular/cdk when they're at that point.

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