fix(chip): mark onFocus as internal

[willshowell]

No description provided.

[jelbourn]

Adding (focus) to the md-chip element already works since the chip element is the thing that receives focus; why would this need to be an @Output?

[willshowell]

@jelbourn tbh I didn't investigate, I just noticed it stood out in the api docs. Happy to repurpose this PR to removing it entirely.

Name	Description
@Input()selected	Whether the chip is selected.
onFocus	Emitted when the chip is focused.
@Output()select	Emitted when the chip is selected.
@Output()deselect	Emitted when the chip is deselected.
@Output()destroy	Emitted when the chip is destroyed.

[jelbourn]

Ah, I see. It should just be internal (_onFocus) since it's used by the chip-list but not meant to be a public API

[jelbourn]

LGTM

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