[border agent] refactor border agent to use MeshCoP Service state changed callback (#2727)

Author: Irving-cl

src/agent/application.cpp

@@ -255,7 +255,11 @@ void Application::InitRcpMode(void)
     mPublisher->Start();
 #endif
 #if OTBR_ENABLE_BORDER_AGENT
-    mBorderAgent->Init();
+    mHost.SetBorderAgentMeshCoPServiceChangedCallback(
+        [this](bool aIsActive, uint16_t aPort, const uint8_t *aTxtData, uint16_t aLength) {
+            mBorderAgent->HandleBorderAgentMeshCoPServiceChanged(aIsActive, aPort,
+                                                                 std::vector<uint8_t>(aTxtData, aTxtData + aLength));
+        });
 // This is for delaying publishing the MeshCoP service until the correct
 // vendor name and OUI etc. are correctly set by BorderAgent::SetMeshCopServiceValues()
 #if OTBR_STOP_BORDER_AGENT_ON_INIT