[border-router] add random delay before chaning routing policies (#6247)

Also fixes an issue that a border router may adds the OMR prefix of
current network as an external route.

tests/scripts/thread-cert/border_router/test_plat_udp_accessiblity.py

@@ -105,7 +105,7 @@ class TestPlatUdpAccessibility(thread_cert.TestCase):
         self.nodes[reset_device].start()
         self.simulator.go(5)
         self.assertIn(self.nodes[reset_device].get_state(), ['leader', 'router'])
-        self.simulator.go(3)
+        self.simulator.go(5)
 
         if reset_device == server:
             # Reconfigure DHCP6 server if necessary