Add XML documentation for Android.Security.Identity

The `Android.Security.Identity` namespace contains **377** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **405** to **377** (reduced by **28**). Related documentation PRs: [#609](https://github.com/dotnet/android-api-docs/pull/609), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command under docs\xml to enumerate the 377 `To be added` entries in Android.Security.Identity source XML documentation, excluding docs\xml\index.xml. Replace the placeholders with appropriate documentation across the matched files, then rerun the scan to confirm none remain.

Written by the indexing model from the issue text.