[google_adsense] Tighten exports and docs. (#8233)
* **Breaking changes**: Reshuffles API exports:
* Removes the adUnit method, and instead exports the AdUnitWidget directly.
* Renames experimental/google_adsense to experimental/ad_unit_widget.dart.
* Removes the AdStatus and AdUnitParams exports.
* Removes the "stub" files, so this package is now web-only and must be used
through a conditional import.
* Tweaks several documentation pages to remove references to internal APIs.
* Splits tests to reflect the new code structure.
## Issue
* Continuation of: https://github.com/flutter/packages/pull/6871
* Part of: https://github.com/flutter/flutter/issues/40376