Add XML documentation for Android.App.SdkSandbox

The `Android.App.SdkSandbox` namespace contains **191** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **187** to **191** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command and reviewing the matching files under docs\xml\Android.App.SdkSandbox and ns-Android.App.SdkSandbox.xml. Replace the 191 reported "To be added" placeholders with appropriate XML documentation, then rerun the scan to confirm that no placeholders remain outside docs\xml\index.xml.

Rédigé par le modèle d'indexation à partir du texte de l'issue.