fix(slide-toggle): not updating model from toggle method

[crisbeto]

Fixes the slide toggle not propagating its value to the ControlValueAccessor when its value is changed through the toggle method.

Fixes #11812.

[devversion]

I think we intentionally didn't emit the event when someone programmatically calls toggle() or changes the checked value, because native input element's don't do either.

[crisbeto]

We definitely need to call at least onChange, otherwise it won't update the ngModel. Also judging by the should support subscription on the change observable test, it seems like we were expecting it to be emitted.

[devversion]

Yeah, I think calling onChange to propagate the change to the ControlValueAccessor makes sense. Although, I'm not sure whether we should really emit the change event. Otherwise we might need to also take into account, that people just change the checked property.

[jelbourn]

We shouldn't emit a change event unless there was a user interaction, similar to the native checkbox. I think mat-checkbox behaves this way.

[crisbeto]

Changed.

[devversion]

LGTM now.

[jelbourn]

LGTM

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