Add XML documentation for Android.Service.Chooser

The `Android.Service.Chooser` namespace contains **195** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **108** to **195** (increased by **87**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml, using the provided rg command to locate the 195 placeholders under Android.Service.Chooser and ns-Android.Service.Chooser.xml. Review the surrounding API declarations and existing documentation patterns, then replace the placeholders with accurate XML documentation; done means the targeted scan no longer reports those placeholders.

Scritto dal modello di indicizzazione a partire dal testo della issue.