Set JAVA_HOME env var to OpenJDK 8 for Android builds
Set JAVA_HOME env var to OpenJDK 8 for Android builds

The new gradle builds require Java 8,
and the existing ant builds also work with Java 8.

This is easier than running many `update-alternatives` calls from Salt.
Moreover, this allows keeping Java 7 installed together with Java 8.

rwaweber/saltfs,servo/saltfs,rwaweber/saltfs,servo/saltfs,rwaweber/saltfs