feat(material/chip): Add focus indicator

[zelliott]

Background: The chip's focus indicator needs to be offset from the chip itself in order to be sufficiently contrastive because the chip can have a primary/accent fill. However, given mat-chip has overflow: hidden, any offset focus indicator will be hidden. The reason mat-chip has its overflow hidden is because it also acts as a ripple target. Thus, in order to add a focus indicator to chip, this PR does the following:

• Instead of using the mat-chip as the ripple target, dynamically create an element and add it as a child node of the mat-chip. This element will be the chip's new ripple target. This element will have its overflow hidden, and the mat-chip will have that style removed.
• Add the .mat-focus-indicator class to the mat-chip.

[zelliott]

Can I accept the new api golden?

[jelbourn]

LGTM

[jelbourn]

If only all CSS was commented this well.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
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[jelbourn]

You can go ahead and make the public API gold update and I can merge the PR

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