Add XML documentation for Android.App.Blob

The `Android.App.Blob` namespace contains **63** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **59** to **63** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan against docs/xml to list the 63 Android.App.Blob XML documentation placeholders, excluding docs/xml/index.xml. Review the matching Android.App.Blob source XML files and replace each placeholder with suitable documentation. Done means the targeted scan no longer reports any of those placeholders.

Vom Indexierungsmodell aus dem Issue-Text verfasst.