Save m.local_notification_settings.<device-id> event in account_data

[mnaturel]

Your use case

What would you like to do?

When background sync is used, on session start and each time we switch on background sync, if no event m.local_notification_settings.<device-id> exists, create and save a new one initialized with the current notification setting.
When switching from background sync to regular push notifications method, delete the created account data event.

Why would you like to do it?

To support remote toggle of push notifications for sessions using background sync.

How would you like to achieve it?

Adding a new check on the start of a session and when changing notification method.

Have you considered any alternatives?

No response

Additional context

No response

Are you willing to provide a PR?

Yes