Return proper HTTP responses in webview testapp

Current Flask throws an exception if a view function returns `None`:

```
python  : TypeError: The view function for 'vibrate' did not return a
valid response. The function either returned None or ended without a
return statement.
```

Maybe that was different in the past, but instead a proper response can
be returned. An empty 200 would be fine, but a 204 No Content response
is used since it better represents what's happening.

Author: dbnicholson

testapps/on_device_unit_tests/test_app/app_flask.py

@@ -110,6 +110,7 @@ def loadUrl():
     print('asked to open url', args['url'])
     activity = get_android_python_activity()
     activity.loadUrl(args['url'])
+    return ('', 204)
 
 
 @app.route('/vibrate')
@@ -122,7 +123,8 @@ def vibrate():
     if 'time' not in args:
         print('ERROR: asked to vibrate but without time argument')
     print('asked to vibrate', args['time'])
-    return vibrate_with_pyjnius(int(float(args['time']) * 1000))
+    vibrate_with_pyjnius(int(float(args['time']) * 1000))
+    return ('', 204)
 
 
 @app.route('/orientation')
@@ -135,4 +137,5 @@ def orientation():
         print('ERROR: asked to orient but no dir specified')
         return 'No direction specified '
     direction = args['dir']
-    return set_device_orientation(direction)
+    set_device_orientation(direction)
+    return ('', 204)