While Kotlin is nowhere to be seen on Android SDK and Google is officially silent on its use.
This matters a lot to customers IT departments, writing the set of delivery languages on their Requests For Proposal to consulting companies.
Kotlin is full interoperable with Java APIs and libraries and at this point there's really very few cases where it's not a significantly better choice for Android development.
It won't, that's what they addressed in this post by ditching jack and switching back to javac bytecode.
"Over time, we realized the cost of switching to Jack was too high for our community when we considered the annotation processors, bytecode analyzers and rewriters impacted."
http://scade.io or chat slack channel perfect.ly to chat with the team.