launcher/adk: merge the package into launcher

[rakyll]

launcher/adk isn't fully following the Go package organization style. The Config could have better suited into the launcher package.

Consider merging launcher/adk into launcher.

[dpasiukevich]

Definitely!

I also wanted to take a proper look and update/improve the launcher package if there'd be any ideas from my side. Config is one of such examples.

[kdroste-google]

Done by @rakyll in #252