Add "#server" as dataplane target value for xDS enabled gRPC servers. (#11715)

As mentioned in [A71 xDS Fallback]( https://github.com/grpc/proposal/blob/master/A71-xds-fallback.md#update-csds-to-aggregate-configs-from-multiple-xdsclient-instances):
updated dataplane target to "#server" for xDS-enabled gRPC servers.

Author: DNVindhya

xds/src/main/java/io/grpc/xds/XdsServerWrapper.java

@@ -172,9 +172,7 @@ public void run() {
 
   private void internalStart() {
     try {
-      // TODO(dnvindhya): Add "#server" as "grpc.target" attribute value for
-      // xDS enabled servers.
-      xdsClientPool = xdsClientPoolFactory.getOrCreate("", new MetricRecorder() {});
+      xdsClientPool = xdsClientPoolFactory.getOrCreate("#server", new MetricRecorder() {});
     } catch (Exception e) {
       StatusException statusException = Status.UNAVAILABLE.withDescription(
               "Failed to initialize xDS").withCause(e).asException();