<amp-web-push> issue going from "Block" to "Allow" push permission

## What's the issue?
There is currently a bug (perhaps a limitation) where manually going from "Block" to "Allow" will not correctly reflect the updated push permission on the main page. This leads to issues such as custom prompting widgets not dismissing or simply continuing to appear altogether despite the permission already set to "Allow"

## How do we reproduce the issue?

[Demo Site](https://amp.dev/documentation/examples/components/amp-web-push/preview/?format=websites)

1. Get prompted for push and accept inside the pop up window
2. Manually change the permission in the browser to "Block"
3. Reload the page
4. Manually change the permission in the browser to "Allow"
5. Reload the page
6. See that the main site still thinks notifications are blocked

To fix it in its current state:

7. Visit [amp-web-push-permission-dialog.html](https://amp.dev/documentation/examples/components/amp-web-push/amp-web-push-permission-dialog.html)
8. Refresh the demo page
9. See that it no longer thinks its blocked
10. You still have to re-subscribe by clicking the button

## What browsers are affected?

Chrome.

Firefox doesn't work at all do to the new user-gesture requirement causing the programmatic native permission request from failing and flickering the popup before it gets dismissed. (This is another issue altogether)

## Which AMP version is affected?

I suspect this has always been broken.