Flutter is increasingly moving to Kotlin rather than Groovy for gradle files; the app template has already switched, and #161703 tracks changing the plugin template.

For all the reasons this is a good idea in flutter/flutter and the templates (better safety, reducing languages we all deal with, better matching standard examples in Android documentation, etc.) it would be good to migrate our existing files in flutter/packages as well.

Per discussion with @reidbaker, we should follow the advice in https://developer.android.com/build/migrate-to-kotlin-dsl to start by standardizing our Groovy files on the more Kotlin-compatible versions of syntax, with things like using = for property assignment and (...) for method calls.