Bug#855262: Set System Encoding as UTF-8

[殷啟聰]

Package: gradle-debian-helper
Version: 1.5.1
Severity: wishlist

On Debian Buildd the default encoding on Java is usually not UTF-8,
which results in compilation failure on source files containing CJK
symbols or even a "©".

Currently we need to add `-Dfile.encoding=UTF-8` to
`override_dh_auto_build`. Why not let gradle-debian-helper do it?

Looks like javadoc does not recognize this variable, we may need to
configure it in the plugin as well.