Add XML documentation for Android.Test.Mock

The `Android.Test.Mock` namespace contains **360** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **356** to **360** (increased by **4**). Related documentation PRs: [#490](https://github.com/dotnet/android-api-docs/pull/490), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the Android.Test.Mock source XML documentation under docs/xml and run the provided rg scan, excluding docs/xml/index.xml, to locate all 360 placeholders. Replace the "To be added" entries with accurate documentation, then rerun the scan; done means no placeholders remain in the targeted namespace.

Written by the indexing model from the issue text.