Add XML documentation for Android.Drm

The `Android.Drm` namespace contains **71** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **69** to **71** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided ripgrep command against docs\xml and inspect the matching Android.Drm/** and ns-Android.Drm.xml files. Replace the 71 `To be added` placeholders with appropriate XML documentation, then rerun the scan to confirm none remain.

Written by the indexing model from the issue text.