Add XML documentation for Android.Systems

The `Android.Systems` namespace contains **1,091** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,118** to **1,091** (reduced by **27**). Related documentation PRs: [#412](https://github.com/dotnet/android-api-docs/pull/412), [#509](https://github.com/dotnet/android-api-docs/pull/509), [#563](https://github.com/dotnet/android-api-docs/pull/563), [#582](https://github.com/dotnet/android-api-docs/pull/582), [#588](https://github.com/dotnet/android-api-docs/pull/588), [#644](https://github.com/dotnet/android-api-docs/pull/644).

针对 docs\xml 运行提供的 rg 扫描，将范围限制为 Android.Systems/** 和 ns-Android.Systems.xml，然后检查匹配的源 XML 文档条目。将识别出的 `To be added` 占位符替换为适用于 Android.Systems API 的适当文档。对于所涵盖的条目，定向扫描不再报告占位符即表示完成。

由索引模型根据 Issue 内容生成。