ahat: Target Java 1.7.

So that users can run ahat with Java 7 without getting unsupported
major/minor version errors.

Change-Id: I76faab19bc1b14c76d5fd6b3ba19fcb07855dfd9
Test: m ahat-test
Test: /usr/local/buildtools/java/jdk7/bin/java -jar ahat.jar
Test: /usr/local/buildtools/java/jdk8/bin/java -jar ahat.jar
Bug: 28303627
2 files changed