2.12 doesn't include any (non-gated) changes though, so you can keep the same codebase and cross-build for 2.12 (for performance) and 2.11. So we're talking what, at least 3 years ("Aida", either 2.13 or 3.0) before Java 8 becomes a blocker for source-level features, at which point I imagine Android will have an answer.
I doubt Google team will change, given their attitude regarding Java 8, as demonstrated at both Google IO 2014 and 2015.
Also the way they are cherry picking features from OpenJDK 7 and some kind of lambda support.
For me, Android Java has become similar to Symbian C++ in terms of dev experience and language fork.
No Android will switch to newer OpenJDK so Scala 2.12 would be a valid target.
Spend some time reading the Android source code.
According to Jake Warton, they only cherry picked some parts of OpenJDK 7.
From the commits they also don't appear to ever fully support Java 8, only lambdas.
ever is a strong word. I know that at the moment they use some parts of OpenJDK 7, but If they need to comply the Java (tm) caused by licensing, they will be forced (sooner or later) to actually don't use a deprecated JVM. As I know OpenJDK will be depracted on 2024. so it will take some time, but it will come.
Using Scala on Android requires a bit more tinkering with ProGuard than Kotlin does, and it isn't a focus of the Scala designers.
Also with Scala 2.12 going Java 8, one will be stuck with 2.11 for Android.