Add XML documentation for Android.Service.QuickSettings

The `Android.Service.QuickSettings` namespace contains **56** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **54** to **56** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to review the 56 placeholders under Android.Service.QuickSettings and the related namespace XML. Replace the “To be added” documentation for the namespace members, then rerun the scan to confirm no placeholders remain in the targeted files.

Vom Indexierungsmodell aus dem Issue-Text verfasst.