Plugin needs to be updated to Flutter's v2 Android plugins API

[xster]

Flutter's Android APIs have been updated in 2019 to better support add-to-app, background execution etc and the majority of Flutter's users are running the new APIs currently.

https://flutter.dev/go/android-plugin-migration

There may currently be issues running the old APIs if, for instance, the plugin is used in the background where there wouldn't be an Activity to provide a context.