Make JAVA_HOME absolute inside build

Export JAVA_HOME as an absolute path to commands running inside
a build.

Should fix errors invoking gradle from inside a build:
ERROR: JAVA_HOME is set to an invalid directory: prebuilts/jdk/jdk8/linux-x86
Please set the JAVA_HOME variable in your environment to match the
location of your Java installation.

Test: treehugger
Change-Id: I16e4482b2d74ede0843715be3b08c65ce33cf403