use App.instance() in kernel launchers

This way later calls to App.instance() will return the running
application, whereas calling App() would create an isolated
Application object.

IPython/zmq/ipkernel.py

@@ -664,7 +664,7 b' def launch_kernel(*args, **kwargs):'
 
 def main():
     """Run an IPKernel as an application"""
-    app = IPKernelApp()
+    app = IPKernelApp.instance()
     app.initialize()
     app.start()
 

IPython/zmq/pykernel.py

@@ -272,7 +272,7 b' def launch_kernel(*args, **kwargs):'
 
 def main():
     """Run a PyKernel as an application"""
-    app = KernelApp()
+    app = KernelApp.instance()
     app.initialize()
     app.start()