Add XML documentation for Android.Security.Keystore

The `Android.Security.Keystore` namespace contains **478** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **469** to **478** (increased by **9**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml, using the provided rg command with the Android.Security.Keystore/** and ns-Android.Security.Keystore.xml globs. Review the 478 source XML documentation entries that still say "To be added", excluding docs\xml\index.xml. Done means the namespace scan no longer reports those placeholders and the documentation remains valid.

Rédigé par le modèle d'indexation à partir du texte de l'issue.