Adds unique ID to Roku for entity registry inclusion (#14325)

* Adds unique ID to Roku for entity registry inclusion

* Owner-requested changes

homeassistant/components/media_player/roku.py

@@ -146,6 +146,11 @@ class RokuDevice(MediaPlayerDevice):
         """Flag media player features that are supported."""
         return SUPPORT_ROKU
 
+    @property
+    def unique_id(self):
+        """Return a unique, HASS-friendly identifier for this entity."""
+        return self.device_info.sernum
+
     @property
     def media_content_type(self):
         """Content type of current playing media."""