fix(radio): host element unable to receive focus events

[crisbeto]

In #13323 we started clearing the tabindex from the host element in order to avoid having two elements with the same tabindex when the consumer sets a custom tabindex. As a result, the mat-radio-button can no longer receive focus and forward it to the input. These changes always reset the tabindex to -1 so the element is focusable, but not tabbable.

Fixes #13953.

Note: this is a resubmit of #13956.

[mmalerba]

Caretaker note: causes a handful of failures in g3 due to assertions about tabindex that are no longer true

[ngbot]

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