Updates the tooling to fetch and use `ktfmt` for Kotlin code, the same way it currently does for the Google Java formatter, and applies that formatting (in the default mode; see discussion in the linked issue) to the repository.
In the future we could revisit the formatter or mode, but since this currently seems to be the most consistent with our other languages and to google3 formatting this is likely the option we'll want to stick with.
Fixes https://github.com/flutter/flutter/issues/118756
72de224b08