Add XML documentation for Android.Util

The `Android.Util` namespace contains **813** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **817** to **813** (reduced by **4**). Related documentation PRs: [#376](https://github.com/dotnet/android-api-docs/pull/376), [#391](https://github.com/dotnet/android-api-docs/pull/391), [#397](https://github.com/dotnet/android-api-docs/pull/397), [#561](https://github.com/dotnet/android-api-docs/pull/561), [#587](https://github.com/dotnet/android-api-docs/pull/587), [#644](https://github.com/dotnet/android-api-docs/pull/644).

首先针对 docs\xml 运行提供的 rg 扫描，然后检查 Android.Util 和 ns-Android.Util.xml 下匹配的文件。将已识别的 `To be added` 占位符替换为适当的 XML 文档，并重新运行扫描以确认占位符数量减少。

由索引模型根据 Issue 内容生成。