xds: Remove newlines from XdsConfig.toString()

The newlines can make logs really hard to read, as many log handling
systems will split up each log statement by line. With the newlines,
those singular log entries get split between multiple log entries which
can make reading/processing it pretty ugly.

When logging XdsConfig by itself, it isn't necessarily _that_ bad when
XdsConfig is what you're actually trying to look at, but when XdsConfig
is in the attributes of ResolvedAddresses it gets very distracting.

Author: ejona86

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

@@ -18,6 +18,7 @@
 
 import static com.google.common.base.Preconditions.checkNotNull;
 
+import com.google.common.base.MoreObjects;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import io.grpc.StatusOr;
@@ -76,14 +77,12 @@ public int hashCode() {
 
   @Override
   public String toString() {
-    StringBuilder builder = new StringBuilder();
-    builder.append("XdsConfig{")
-        .append("\n  listener=").append(listener)
-        .append(",\n  route=").append(route)
-        .append(",\n  virtualHost=").append(virtualHost)
-        .append(",\n  clusters=").append(clusters)
-        .append("\n}");
-    return builder.toString();
+    return MoreObjects.toStringHelper(this)
+        .add("listener", listener)
+        .add("route", route)
+        .add("virtualHost", virtualHost)
+        .add("clusters", clusters)
+        .toString();
   }
 
   public LdsUpdate getListener() {