gnu: Add and use default IcedTea.

* gnu/packages/java.scm (icedtea): New variable.
(swt)[native-inputs]: Replace "icedtea6" with "icedtea".
* gnu/packages/bioinformatics.scm (htsjdk)[native-inputs]: Likewise.
(ngs-java)[inputs]: Likewise.
* gnu/packages/kodi.scm (kodi)[native-inputs]: Replace "icedtea7" with
"icedtea".
* gnu/packages/ruby.scm (ruby-rjb)[native-inputs]: Replace "icedtea7"
with "icedtea".