Add XML documentation for Android.Security

The `Android.Security` namespace contains **303** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **297** to **303** (increased by **6**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided ripgrep command first to list the 303 placeholders under docs\xml in the Android.Security paths, excluding docs\xml\index.xml. Review the matching source XML documentation entries and replace the placeholders; done means the scan no longer reports those entries.

Written by the indexing model from the issue text.