Bug#801510: fdroidserver: Hardcoded openJDK7

[Hans-Christoph Steiner]

fdroidserver needs syntax for keytool and jarsigner that was added in Java 1.7
that allows sending passwords via file on the file system or via stdin. That
version or newer is now default on Debian/jessie and newer, and Ubuntu/trusty
and newer, so I switched this package to default-jdk and let it use OpenJDK 7,
8, or 9 paths.