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.

索引モデルが issue の本文から書いたものです。