Add XML documentation for Android.Service.Controls

The `Android.Service.Controls` namespace contains **436** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **434** to **436** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided PowerShell rg command against docs\xml to list the 436 placeholders under Android.Service.Controls, excluding docs\xml\index.xml. Read the matched source XML documentation and the related changes in PR #644 before editing. Done means the namespace's placeholders have been replaced with documentation and the scan no longer reports them.

索引モデルが issue の本文から書いたものです。