Different versions of Ubuntu ship with different versions of Java.
Trusty had 7, Xenial has 8, and so on. This causes problems when we
hardcode a versioned package name into our dep lists as that version may
not exist everywhere. Thankfully Ubuntu provides a default-jre-headless
package that we can use instead that maps properly onto whatever java
version is correct.
Change-Id: I4e5da215c8f7aa426494686d5043995ce5d3c3af
a09cb5acf9