Hello,
The latest Jenkins LTS release, 1.625, requires Java7 to run (to give some context, Java9 is in beta now, Java7 is many years old). As learned with some pain on ci.linaro.org, which already upgraded, this requirement applies not just to a Jenkins master, but also to build slaves, just to run slave agent to be connected to a master.
We install both Java6 and Java7 on android-build.linaro.org slaves, but the default is still Java6, "for compatibility with older Android versions", as a comment says.
Is Java6 being a default still relevant? The easiest way to accommodate the upcoming upgrade is just to switch default to Java7. If there're issues expected with that, we can follow an alternative way of adding adhoc version overrides for slave startup.