xds: Use load_balancing_policy if provided in Cluster.

Renames the LegacyLoadBalancerConfigFactory to just LoadBalancerConfigFactory and gives it responsibility for both the legacy and the new LB config mechanism.

The new configuration mechanism is exaplained in gRFC A52: grpc/proposal#298

Author: temawi

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

@@ -1635,9 +1635,7 @@ static CdsUpdate processCluster(Cluster cluster, Set<String> retainedEdsResource
     }
     CdsUpdate.Builder updateBuilder = structOrError.getStruct();
 
-    // TODO: If load_balancing_policy is set in Cluster use it for LB config, otherwise fall back
-    // to using the legacy lb_policy field.
-    ImmutableMap<String, ?> lbPolicyConfig = LegacyLoadBalancerConfigFactory.newConfig(cluster,
+    ImmutableMap<String, ?> lbPolicyConfig = LoadBalancerConfigFactory.newConfig(cluster,
         enableLeastRequest);
 
     // Validate the LB config by trying to parse it with the corresponding LB provider.