[amp-consent] Post-prompt UI doesn't appear in sample

## What's the issue?

In [the new granular amp-consent sample on amp.dev](https://amp.dev/documentation/examples/user-consent/granular_user_consent), the post-prompt UI never appears.

## Briefly describe the bug.

It doesn't appear! Its `hidden` attribute is never removed:

```html

However, it does appear in [the demo version with the same code](https://amp.dev/documentation/examples/user-consent/granular_user_consent/preview/?format=websites), as well as [the playground version](https://playground.amp.dev/?url=https%3A%2F%2Fpreview.amp.dev%2Fdocumentation%2Fexamples%2Fuser-consent%2Fgranular_user_consent&format=websites). I can only surmise that the failure in the first version is caused by the DOM structure that surrounds the `` component there.

I kept looking for something I'd done wrong, but I haven't found that something. (You might. 😏)

Start with the granular amp-consent sample on amp.dev and compare its DOM structure with the linked demo and playground versions, focusing on the repromptDialog element and amp-consent component. Done means the post-prompt UI appears in the sample and its hidden attribute is removed as it is in the working versions.

Written by the indexing model from the issue text.