Add app_id property to Fire TV component (#20719)

homeassistant/components/media_player/firetv.py

@@ -171,6 +171,11 @@ class FireTVDevice(MediaPlayerDevice):
         """Return whether or not the ADB connection is valid."""
         return self._available
 
+    @property
+    def app_id(self):
+        """Return the current app."""
+        return self._current_app
+
     @property
     def source(self):
         """Return the current app."""