Support UnifiedPush on Android

[Zk2u]

UnifiedPush is a protocol used in apps like Amethyst and Molly (Signal client fork) to send push notifications via a server that google doesn't control. Phoenix uses FCM currently for push notifications, but it would be awesome if it also shipped a FOSS build that uses UnifiedPush instead of FCM.

[warioishere]

I would like to push this too. I am using GrapheneOS and I DONT want to install g-play for Push.

Pls provide support for alternative Push Systems like Unified Push so we can be independent from Google and decentralize the Internet too!