Add XML documentation for Android.Graphics.Drawables

The `Android.Graphics.Drawables` namespace contains **575** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **546** to **575** (increased by **29**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to inspect the 575 placeholders under Android.Graphics.Drawables. Read the matching namespace and source XML entries to understand the documentation format, then replace the identified “To be added” placeholders and rerun the scan. Done means no matching placeholders remain in that namespace, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.