Feat: Add Unifiedpush support

[p1gp1g]

Following web push support in nextcloud/notifications, we can add UnifiedPush support to Talk (and Nextcloud Android) to get push notifications even without the Play Services.

The implementation follows this guide: https://unifiedpush.org/developers/ux/

• During first startup, the push notification is setup:
  • If the Play Services are available, they are used by default
  • Else, if UnifiedPush is available, it is used
  • If the app uses UnifiedPush and the user has many distributors, but hasn't selected a default yet (again, nearly never the case - the default is a OS wide default), a dialog is shown to introduce the OS screen.
• Settings:
  • If a distributor is available, you have a new setting that allows to enable/disable UnifiedPush
  • If the user has many distributors (nearly never the case - but that means the user knows what UnifiedPush is!), a 2nd setting is offered, to change the distributor

This feature also gives the possibility to get Push Notifications with the Play Services, without the proxy (Nextcloud servers directly push to Google FCM servers), and without a proprietary library. So this is an accepted way to use FCM for application in F-Droid. => This is in a 2nd PR, already ready.

PS: The description is closed to Nextcloud PR, but the information apply here too

Fix #257

🖼️ Screenshots

🏚️ Before	🏡 After

|

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[p1gp1g]

I got a conflict with my own previous PR 🙈 -> I'm fixing it

[p1gp1g]

Oh, and I've seen something strange during the implementation:

userManager.users.blockingGet() returns 2 users even if I'm connected with a single user. I've tested multiple time, from a fresh install and it always happen. The 2 get the same username, but different userId. Are you aware of that? Should I open an issue ? I think the duplication happens during the registration, but I'm not sure at all

[mahibi]

Thank you so much @p1gp1g i'm really looking forward to have UnifiedPush implemented 👍 👍 👍
We will take a close look as soon as time allows, it might not be this week though.

[mahibi]

Oh, and I've seen something strange during the implementation:

userManager.users.blockingGet() returns 2 users even if I'm connected with a single user. I've tested multiple time, from a fresh install and it always happen. The 2 get the same username, but different userId. Are you aware of that? Should I open an issue ? I think the duplication happens during the registration, but I'm not sure at all

Thanks for reporting! This sounds like #4499 so no need to create a new issue. So yes, we will have to take a closer look how this can happen during registration.