Fix bootstrap class path warning (java)

Due to forcing java 6 we need to pass the
bootstrap jar file (android.jar).
https://blogs.oracle.com/darcy/entry/bootclasspath_older_source

Change-Id: I530a7e2a7df40813011a6dde93d6ccc3aaaa61d6
warning: [options] bootstrap class path not set in conjunction with -source 1.6
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>