Simplify access to service callback (#172)

This also works around the problem of assigning to an 'incompatible type', but looks much cleaner.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

Author: jacobperron

rcljava/src/main/java/org/ros2/rcljava/service/ServiceImpl.java

@@ -100,9 +100,7 @@ public final long getHandle() {
 
   public void executeCallback(
       RMWRequestId rmwRequestId, MessageDefinition request, MessageDefinition response) {
-    TriConsumer<RMWRequestId, MessageDefinition, MessageDefinition> callback =
-        ((ServiceImpl) this).callback;
-
+    TriConsumer callback = this.callback;
     callback.accept(rmwRequestId, request, response);
   }