Add support for locks in google assistant component (#18233)

* Add support for locks in google assistant component

This is supported by the smarthome API, but there is no documentation
for it. This work is based on an article I found with screenshots of
documentation that was erroneously uploaded:

https://www.androidpolice.com/2018/01/17/google-assistant-home-can-now-natively-control-smart-locks-august-vivint-first-supported/

Google Assistant now supports unlocking certain locks - Nest and August
come to mind - via this API, and this commit allows Home Assistant to
do so as well.

Notably, I've added a config option `allow_unlock` that controls
whether we actually honor requests to unlock a lock via the google
assistant. It defaults to false.

Additionally, we add the functionNotSupported error, which makes a
little more sense when we're unable to execute the desired state
transition.

https://developers.google.com/actions/reference/smarthome/errors-exceptions#exception_list

* Fix linter warnings

* Ensure that certain groups are never exposed to cloud entities

For example, the group.all_locks entity - we should probably never
expose this to third party cloud integrations. It's risky.

This is not configurable, but can be extended by adding to the
cloud.const.NEVER_EXPOSED_ENTITIES array.

It's implemented in a modestly hacky fashion, because we determine
whether or not a entity should be excluded/included in several ways.

Notably, we define this array in the top level const.py, to avoid
circular import problems between the cloud/alexa components.

tests/components/cloud/test_iot.py

@@ -326,6 +326,8 @@ def test_handler_google_actions(hass):
         'switch.test', 'on', {'friendly_name': "Test switch"})
     hass.states.async_set(
         'switch.test2', 'on', {'friendly_name': "Test switch 2"})
+    hass.states.async_set(
+        'group.all_locks', 'on', {'friendly_name': "Evil locks"})
 
     with patch('homeassistant.components.cloud.Cloud.async_start',
                return_value=mock_coro()):