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).

Run the provided rg scan against docs\xml, limited to Android.Systems/** and ns-Android.Systems.xml, then review the matching source XML documentation entries. Replace the identified `To be added` placeholders with appropriate documentation for the Android.Systems APIs. Done means the targeted scan no longer reports placeholders for the entries covered.

Vom Indexierungsmodell aus dem Issue-Text verfasst.