feat(xds): Add CachedChannelManager for caching channel instances

Author: sauravzg

xds/src/test/java/io/grpc/xds/internal/grpcservice/CachedChannelManagerTest.java

@@ -76,8 +76,8 @@ private GrpcServiceConfig buildConfig(String target, String credsType) {
         .target(target)
         .configuredChannelCredentials(creds)
         .build();
-        
-    return GrpcServiceConfig.builder()
+
+    return GrpcServiceConfig.newBuilder()
         .googleGrpc(googleGrpc)
         .initialMetadata(ImmutableList.of())
         .build();