Update UnifiedPush Connector library to 3.x
- Update the UnifiedPush connector library to the latest version (3.x.y).
- Check the changes in the UP Connector API and in the AND_3 spec and whether they apply to us.
- Implement a [PushService](https://unifiedpush.org/kdoc/connector/org.unifiedpush.android.connector/-push-service/) ~~that the distributor can bind to to raise FMD to the foreground.~~ EDIT: this is a different service, see [below](https://gitlab.com/fmd-foss/fmd-android/-/work_items/394#note_3154926990).
- Check distributor selection UI: https://codeberg.org/UnifiedPush/android-connector-ui (see the links in the deprecation notice).
Currently [the lib we include](https://gitlab.com/fmd-foss/fmd-android/-/blob/v0.14.0/app/build.gradle?ref_type=tags#L142) is very old:
```gradle
implementation 'com.github.UnifiedPush:android-connector:2.1.1'
```
## References
- https://codeberg.org/UnifiedPush/wishlist/issues/22
- https://codeberg.org/UnifiedPush/specifications/src/branch/main/specifications/android.md
- https://codeberg.org/UnifiedPush/android-connector/releases
- https://unifiedpush.org/kdoc/connector/org.unifiedpush.android.connector/-push-service/
issue