fix(button-toggle): dirty on load when used with ngModel

[crisbeto]

Fixes the button toggle group being marked as dirty on load, if it has a ControlValueAccessor and an initial value.

Fixes #4888.

[tinayuangao]

LGTM

[kara]

@crisbeto It seems to me that the underlying problem is that value changes from any source lead to the forms API onChange function being called (which sets the control to dirty), rather than only user-generated value changes. Deferring the setting of the isInitialized property avoids the initial call, but any programmatic writes after that will still improperly mark the control as dirty. Can we pass a boolean into the _emitChangeEvent method for whether it was user-generated, then only call the forms API change function in that case?

[jelbourn]

@crisbeto is this worth revisiting?

[crisbeto]

Yeah it's been on my TODO list, but I was waiting for the refactoring to get in.

[jelbourn]

Ack, we just have some failing tests in Firebase on the refactor I need to debug. Poke me on this when it's resolved

[josephperrott]

@crisbeto Please rebase when you have a chance.

[crisbeto]

Looks like the original issue got fixed by #9191. Closing.

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