fix(ngModel): test & update correct model when running $validate

If `$validate` is invoked when the model is already invalid, `$validate`
should pass `$$invalidModelValue` to the validators, not `$modelValue`.

Moreover, if `$validate` is invoked and it is found that the invalid model
has become valid, this previously invalid model should be assigned to
`$modelValue`.

Lastly, if `$validate` is invoked and it is found that the model has
become invalid, the previously valid model should be assigned to
`$$invalidModelValue`.

Closes #7836
Closes #7837