Add XML documentation for Android.Hardware.Biometrics

The `Android.Hardware.Biometrics` namespace contains **474** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **404** to **474** (increased by **70**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to enumerate the 474 `To be added` placeholders under Android.Hardware.Biometrics, excluding docs\xml\index.xml. Read the affected source XML files and replace each placeholder with accurate API documentation; done means the scan returns no remaining placeholders in that namespace.

Scritto dal modello di indicizzazione a partire dal testo della issue.