Add XML documentation for Android.Ranging.Uwb

The `Android.Ranging.Uwb` namespace contains **206** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **198** to **206** (increased by **8**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command under docs\xml to enumerate the Android.Ranging.Uwb files and the 206 `To be added` placeholders, excluding docs\xml\index.xml. Read the surrounding XML documentation and relevant Android.Ranging.Uwb declarations before replacing the placeholders. Done means all 206 targeted placeholders have meaningful documentation and the scan no longer finds them.

Written by the indexing model from the issue text.