[nicl] Removed static Context instance and reduced access levels when possible.

[Sundararajan Athijegannathan]

jextract changes:

* Context static instance field removed. Current Context is passed 
around instead.
* Reduced access levels of fields and methods when possible.

Please review: 
http://cr.openjdk.java.net/~sundar/jextract_no_static_context/webrev.00/

Thanks,
-Sundar