Add recipe for developing plugins on Android

[xster]

Via flutter/flutter#44166, we'll start asking plugin developers to create plugins compatible with both old and new Android embeddings their users will use. Since we never documented how to write Java code for creating plugins, https://flutter.dev/docs/development/packages-and-plugins/developing-packages#step-2b-add-android-platform-code-javakt, it would be good to have a breakout for that topic.

As a potential example, add a section in https://flutter.dev/docs/cookbook for Android plugin development with a realistic scenario and walk through how the old PluginRegistry and the new FlutterPlugin APIs work.