Add XML documentation for Android.Runtime

The `Android.Runtime` namespace contains **1,915** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,911** to **1,915** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided ripgrep command against docs\xml to inspect the Android.Runtime entries and their “To be added” placeholders. Replace the placeholders with XML documentation, using the surrounding source documentation as context. Done means the Android.Runtime scan reports no remaining placeholders, excluding docs\xml\index.xml.

索引モデルが issue の本文から書いたものです。