fix(ngAria): handle non-string model values

Fix for #10212. Compare the string value of the model
so that aria-checked will be properly applied to
type=radio in the case where the model is a number

Closes #10212
$render of inputTypeRadio checks if `$viewValue === attr.value`, not the with the modelValue. I checked, that works. We could also expose the check as ctrl.$isEmpty(). Then we could do:

`var boolVal = !ngModel.$isEmpty(ngModel.$viewValue);`

Still a bit silly that we need to pass the $viewValue, but since we now again always use $isEmpty with the $viewValue everywhere (or should), we could change $isEmpty in 1.4 to not take an argument, so we'd have a canonical way of getting the empty state for ngAria and other plugins.