Respond to Alexa scene activation correctly (#11869)

The API documentation[1] specifies that Alexa.SceneController Activate
must get a ActivationStarted response. Responding with just a `Response`
will elicit a "Hmm... $scene is not responding" from Alexa.

[1]: https://developer.amazon.com/docs/smarthome/provide-scenes-in-a-smart-home-skill.html

tests/components/alexa/test_smart_home.py

@@ -739,7 +739,9 @@ def test_api_activate(hass, domain):
 
     assert len(call) == 1
     assert call[0].data['entity_id'] == '{}.test'.format(domain)
-    assert msg['header']['name'] == 'Response'
+    assert msg['header']['name'] == 'ActivationStarted'
+    assert msg['payload']['cause']['type'] == 'VOICE_INTERACTION'
+    assert 'timestamp' in msg['payload']
 
 
 @asyncio.coroutine