Check the bios of many of the major Android team leads, some of them were former Java developers at Sun, which I think has a big weight on Java's role on Android.
As for Fuchsia and Brillo, it remains to be seen if they will ever be productified.
Also I don't believe that Dart is a better option than Java in regards to performance.
I believe Google is running out of solutions to further optimize Android because they keep running into self inflicted Java roadblocks. If Fuchsia is any indication of their future efforts their next major overhaul will support multiple first class languages.