Notifications are not accessible via keyboard

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug

When a notification is received, it is not announced by Orca.

### To Reproduce
Steps to reproduce the behavior:
1. Enable Orca with Alt + Super + S
2. Receive a desktop notification.
3. Observe that Orca does not announce it.
### Expected behavior
Orca should read out the notification, including the fact it is a notification, the app name, and the notification content.
### Screenshots or screen recordings

### Logs

## Platform Information
- OS: elementary OS
- OS Version: Circe

Please check what applies:
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- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version

## Additional context
Gnome's notification daemon sends these correctly, so it might be worth looking there.