Add XML documentation for Android.Service.Settings.Preferences

The `Android.Service.Settings.Preferences` namespace contains **388** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **412** to **388** (reduced by **24**). Related documentation PRs: [#578](https://github.com/dotnet/android-api-docs/pull/578), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to inspect the 388 placeholders under Android.Service.Settings.Preferences and the related namespace XML file. Review the existing documentation patterns in those files and the related PRs #578 and #644. Done means the targeted source XML documentation placeholders have been replaced with appropriate documentation.

Written by the indexing model from the issue text.