Add javac attributes to keep Android-compatibility

When the attributes are absent, the jar files will be build against
whichever Java version the JDK is using, causing problems when using
JDK version greater than 6.

A typical problem caused by this:
http://stackoverflow.com/questions/7866723/dx-bad-class-file-magic-cafebabe-or-version-0033-0000-with-adk14