Agp 9.1 / newdsl follow up work #190964

This issue tracks the work that is discovered as part of the 12 pr stack of work to migrate flutter to newdsl.

See #189407

• Convert docs/platforms/android/website-page-draft.md into a flutter website pr
• Delete docs/platforms/android/Migrating-Flutter-Gradle-Plugin-to-AGP-public-API.md
  • it is a worklog document that the agent used to keep track during its own migration and rebasing but does not belong long term in the flutter codebase.