Rework chromecast fix (#16804)

* Revert changes of #16471, and fix the platform setup issue

* Fix unit test

* Fix

* Fix comment

* Fix test

* Address review comment

* Address review comment

tests/common.py

@@ -609,16 +609,18 @@ def patch_yaml_files(files_dict, endswith=True):
     return patch.object(yaml, 'open', mock_open_f, create=True)
 
 
-def mock_coro(return_value=None):
-    """Return a coro that returns a value."""
-    return mock_coro_func(return_value)()
+def mock_coro(return_value=None, exception=None):
+    """Return a coro that returns a value or raise an exception."""
+    return mock_coro_func(return_value, exception)()
 
 
-def mock_coro_func(return_value=None):
+def mock_coro_func(return_value=None, exception=None):
     """Return a method to create a coro function that returns a value."""
     @asyncio.coroutine
     def coro(*args, **kwargs):
         """Fake coroutine."""
+        if exception:
+            raise exception
         return return_value
 
     return coro