core, grpclb: Enable new pick first LB by default and update grpclb tests

Enable GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST by default in PickFirstLoadBalancerProvider.
Duplicate failing tests in GrpclbLoadBalancerTest to verify both legacy PickFirstLoadBalancer and modern PickFirstLeafLoadBalancer behaviors.

TAG=agy
CONV=2b34d6af-c0df-46b4-8326-2dea3641f8cc

Author: kannanjgithub

core/src/main/java/io/grpc/internal/PickFirstLoadBalancerProvider.java

@@ -37,7 +37,7 @@ public final class PickFirstLoadBalancerProvider extends LoadBalancerProvider {
   private static final String SHUFFLE_ADDRESS_LIST_KEY = "shuffleAddressList";
 
   static boolean enableNewPickFirst =
-      GrpcUtil.getFlag("GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST", false);
+      GrpcUtil.getFlag("GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST", true);
 
   public static boolean isEnabledHappyEyeballs() {