Add XML documentation for Android.App.Slices

The `Android.App.Slices` namespace contains **268** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **262** to **268** (increased by **6**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command in docs\xml to locate the 268 `To be added` placeholders under Android.App.Slices, excluding docs\xml\index.xml. Read the surrounding XML entries and related documentation PR #644 for context. Done means the namespace no longer contains those placeholders and the scan reports none remaining.

Written by the indexing model from the issue text.