It has been way too common for people to accidentally have a javac
from JDK 7 create class files that are of the newest version, which
older JREs and Android's dex then don't understand.

As many of the distro-configs files specified
--with-java-target-version=1.5 anyway, let's just hardcode that for
now.

Change-Id: I0853820dc56a062f053b5c2c30097bf1ff3798c1