I don't think so. In order for that to happen, Google would need to support new bytecodes in their JVM (Dalvik? I forget), which would be a backwards-incompatible change. So unfortunately I don't expect to see Java 8 in Android for quite a while.
You already have to choose between Java 7 and target only 4.4 onwards devices or use Java 6 to be able to target older devices.
They could have released Marshmallow with Java 8 support, so that in the same way one could choose to target 6.0 devices and newer or not.