Replace FCM with UnifiedPush (maybe with embedded FCM)

[genofire]

Is your feature request related to a problem?

I like to get PushNotification from Android without Google (Play) Services / FCM.

Describe the solution you'd like

UnifiedPush offers a alternative Way of push notification.

Describe alternatives you've considered

For non F-Droid Builds, there is the option to use an embedded fcm inside the unified dependence, as fallback.

Additional context

It would be nice, if there is an buildfalvor for

• googleplay with unifiedPush with embedded fcm
• f-droid with unifiedpush

[genofire]

Duplicated of #8684