Add XML documentation for Android.AdServices.Common

The `Android.AdServices.Common` namespace contains **357** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **341** to **357** (increased by **16**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan in docs\xml to inspect the 357 placeholders under Android.AdServices.Common, excluding docs\xml\index.xml. Review the related documentation work in PR #644 and the surrounding XML entries to determine the expected wording. Done means the namespace's source XML documentation no longer contains these “To be added” placeholders.

Vom Indexierungsmodell aus dem Issue-Text verfasst.