Add XML documentation for Android.App.Wallpaper

The `Android.App.Wallpaper` namespace contains **142** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **138** to **142** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 142 placeholders under Android.App.Wallpaper and ns-Android.App.Wallpaper.xml. Read the surrounding API entries and existing documentation patterns before updating each "To be added" entry. Done means the namespace’s source XML documentation no longer contains those placeholders, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.