interop-testing: Start mgmt server before xds server

The xds server can take a really long time to start if the xds resources
are slow to load. Ideally the management server would be available
during this time so we can inspect the server. The server health still
won't go to SERVING until the xds server starts, which is appropriate.

Author: ejona86

interop-testing/src/main/java/io/grpc/testing/integration/XdsTestServer.java

@@ -176,14 +176,6 @@ private void start() throws Exception {
     }
     health = new HealthStatusManager();
     if (secureMode) {
-      server =
-          XdsServerBuilder.forPort(
-                  port, XdsServerCredentials.create(InsecureServerCredentials.create()))
-              .addService(
-                  ServerInterceptors.intercept(
-                      new TestServiceImpl(serverId, host), new TestInfoInterceptor(host)))
-              .build();
-      server.start();
       maintenanceServer =
           NettyServerBuilder.forPort(maintenancePort)
               .addService(new XdsUpdateHealthServiceImpl(health))
@@ -192,6 +184,14 @@ private void start() throws Exception {
               .addServices(AdminInterface.getStandardServices())
               .build();
       maintenanceServer.start();
+      server =
+          XdsServerBuilder.forPort(
+                  port, XdsServerCredentials.create(InsecureServerCredentials.create()))
+              .addService(
+                  ServerInterceptors.intercept(
+                      new TestServiceImpl(serverId, host), new TestInfoInterceptor(host)))
+              .build();
+      server.start();
     } else {
       server =
           NettyServerBuilder.forPort(port)