use jarsigner and keytool from same JDK as is being set in JAVA7_HOME

Using the same JDK throughout should prevent weird bugs where a setup might
use Java8's jarsigner and Java7's keytool.  This also allows the user to
set java_paths and have jarsigner and keytool used from that specified JDK.

This incorporates almost all of the patch that is in the Debian package
that forces fdroidserver to use the default JDK on that Debian release.

closes #93 fdroid/fdroidserver#93
2 jobs for auto-detect-java-homes
Status Job ID Name Coverage
  Test
passed #685919
test

00:25:37

passed #685915
test

00:25:56