Pin apache-harmony-tests to Java 1.7 / v52

The switch to v52 broke jarjar. jarjar just silently
ignores v52 class files right now. That meant that these
tests stopped running.

This pinning is to get things working. It should be
removed once jarjar is fixed or is removed for other
reasons.

Bug: 27637680
Change-Id: I37f2a54a5113e841e7e46b91e382fc474f7bfcfd