feat(checkbox): expose ripple instance

[devversion]

• Exposes the reference to the MatRipple directive publically.

Similar to #9170. References #4179

[jelbourn]

This seems out of sync with the discussion in #4179, no?

[devversion]

The PR should resolve #4179. It's not the implementation of the last idea (with the mixins), which intends to provide another public API for the ripples.

In my opinion, if we should expose the ripple instance, then it should be really the MatRipple and no wrapper, that just provides a nicer API.

Since there is a overview file for the ripple now, I think that it would be fine just exposing the instance. Also, as I mentioned in #9170, we can add a guide that shows how to control ripples of components that expose the instance.

[jelbourn]

LGTM

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