core: Validate load-balancing policy name early in ManagedChannelBuilder (#12695)

Author: themechbro

api/src/main/java/io/grpc/ManagedChannelBuilder.java

@@ -15,7 +15,6 @@
  */
 
 package io.grpc;
-
 import com.google.common.base.Preconditions;
 import java.util.List;
 import java.util.Map;

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

@@ -213,7 +213,6 @@ public static ManagedChannelBuilder<?> forTarget(String target) {
   private boolean recordRetryMetrics = true;
   private boolean tracingEnabled = true;
   List<MetricSink> metricSinks = new ArrayList<>();
-
   /**
    * An interface for Transport implementors to provide the {@link ClientTransportFactory}
    * appropriate for the channel.