Add XML documentation for Android.Service.Carrier

The `Android.Service.Carrier` namespace contains **180** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Service.Carrier/**' --glob 'ns-Android.Service.Carrier.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **176** to **180** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 180 placeholders under Android.Service.Carrier, excluding docs\xml\index.xml. Read the matching XML entries and related API context before updating them. Done means the namespace's placeholders are replaced with accurate documentation and the scan no longer reports them.

Written by the indexing model from the issue text.