java: set context class loader of context's main thread.

This is an optimization which saves the isolate code from going
through OsvSystemClassLoader proxy.

java/runjava/src/main/java/io/osv/ContextIsolator.java

@@ -114,6 +114,7 @@ public class ContextIsolator {
                 e.printStackTrace();
             }
         });
+        thread.setContextClassLoader(classLoader);
         thread.start();
         return context;
     }