Add -sigalg and -digestalg options for jarsigner to be able to sign an apk in jdk 7.

The default digest algorithm of jarsigner for Java 1.7 is SHA256withRSA which isn't supported in Android, while for Java 1.6 it's MD5withRSA
So "-sigalg SHA1withRSA -digestalg SHA1" options are needed to sign correctly in jdk 7.

See this discussion for more details.
http://stackoverflow.com/questions/8739564/what-is-the-difference-between-the-java-1-6-and-1-7-jarsigner