Push Notifications not working in Firefox due to user gesture requirement

### Description

Due to Firefox's user-gesture requirement described [here](https://developer.mozilla.org/en-US/docs/Web/API/Push_API/Best_Practices#browser_mitigations), AMP push notifications are currently broken on Firefox sites.

When attempting to subscribe in Firefox, you will see the popup flash. This is due to the window opening and immediately attempting to trigger the native notification prompt. However, since this is currently being done programmatically, the prompt fails and the window gets closed.


### Reproduction Steps

1. Open [the demo site](https://amp.dev/documentation/examples/components/amp-web-push/preview/?format=websites) in Firefox
2. Attempt to subscribe to notifications

### Relevant Logs

_No response_

### Browser(s) Affected

Firefox

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start with the amp-web-push demo site in Firefox and reproduce the subscription flow described in the issue. Trace the amp-web-push implementation responsible for opening the window and triggering the native notification prompt, then verify that subscribing no longer closes the window or fails because of Firefox's user-gesture requirement.

Written by the indexing model from the issue text.