bug(ngAria): Use of ng-true-value and ng-false-value not updating aria-checked

[marcysutton]

Checkbox components using ng-true-value and ng-false-value are not computing the correct aria-checked value.

See this thread from Angular Material: angular/material#882 (comment)

Example code:

<md-switch ng-model="data.cb2" aria-label="Switch 2" ng-true-value="'yup'" 
ng-false-value="'nope'" role="checkbox" tabindex="0" 
aria-checked="true" aria-invalid="false">
</md-switch>

When the checkbox is toggled to the off position using these attributes, the aria-checked property is set to true. It should be false.