[thci] use 'br state' to check isBorderRoutingEnabled (#13506)

tools/harness-thci/OpenThread.py

@@ -3171,8 +3171,7 @@ class OpenThreadTHCI(object):
     @watched
     def isBorderRoutingEnabled(self):
         try:
-            self.__executeCommand('br omrprefix local')
-            return True
+            return self.__executeCommand('br state')[0] in ('running', 'stopped')
         except CommandError:
             return False