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.

Written by the indexing model from the issue text.