If/when Google decides it's time to sanction a Java replacement on Android, the language designed by their tool provider seems like the obvious choice.
The biggest change was to switch to gradle which is a more powerful build system but also much buggier (at least in the current Android Studio). They also lots of useful Android-related tools and lints, and a fair helping of UI bugs.
Still, beats Eclipse by a country mile.
As a standard, a common denominator, Java the language is a good choice because it is popular. Other languages running on top of the JVM can be used on top of Android as well, mostly anyway, at least if you refrain from generating and interpreting bytecode at runtime.
One of the main advantages of Kotlin over Scala on Android is that Kotlin is a small increment over Java with very little jar size and language overhead.
Personally I do not care about how writing the actual code makes me feel or whatever, what I care about is how well the final artifact works versus the effort I've put in making it work. The act of learning and using a new language is completely useless if the new language doesn't make possible and/or comfortable new abstractions that help me in managing complexity (that would be otherwise not possible or very painful in Java).
Therefore personally I'll take languages like Scala or Clojure any day, because in spite of the learning experience being painful, at least I got something out of it.
N.B. - I don't really know Kotlin, I only played with it and I don't have an opinion on it yet - might turn out to be a good language for me, but I hope this will not be the reason promoted for using it, because it's a very poor reason. Granted, it could be popular, just like CoffeeScript, but then again, I also hate CoffeeScript.
On the small JAR size - granted that's useful, but Android apps tend to be compressed for distribution by means Proguard and Pack2000, which does tree-shaking, not perfect but theoretically you pay for what you use and in practice it works very well, not being worse than the result of importing dependencies in your project like from Apache Commons or Google's Guava or whatever and paying a couple hundred KBs for it - which counts, but nothing tragic.