Accordion isExpanded

We want to be able to delay rendering until the accordion is expanded. Currently, the isExpanded is never updated when the accordion item is expanded.

If this is updated in the template we can use the isExpanded value with the ngIf angular directive to delay rendering until the accordion is expanded.

<usa-accordion #accordion>
  <component *ngIf="accordion.isExpanded"> ... </component>
</usa-accordion>