Feature Request: Support Unified Push on Android

[delitako]

Zulip already has to jump through hoops to support push notifications through Google. Even with this, mobile notifications do not work when using a degoogled Android phone. It would be awesome if UnifiedPush were supported. The spec seems relatively simple and it requires no special solutions to work with self-hosted instances. However, it does require support from both the client and server.

[devurandom]

See-also: https://github.com/zulip/zulip-mobile/issues/3838