Move GCP proxy code to the old [] proxy's location

1. Moved code for the GCP proxy to where the [] proxy code used to live.
3. Corrected reference to the GCP proxy location.
4. Misc changes to make ErrorProne and various tools happy.

+diekmann to LGTM terraform whitelist change.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213630560

java/google/registry/proxy/handler/HealthCheckHandler.java

@@ -0,0 +1,43 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.proxy.handler;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.channel.ChannelFuture;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelInboundHandlerAdapter;
+import java.nio.charset.StandardCharsets;
+
+/** A handler that responds to GCP load balancer health check message */
+public class HealthCheckHandler extends ChannelInboundHandlerAdapter {
+
+  private final ByteBuf checkRequest;
+  private final ByteBuf checkResponse;
+
+  public HealthCheckHandler(String checkRequest, String checkResponse) {
+    this.checkRequest = Unpooled.wrappedBuffer(checkRequest.getBytes(StandardCharsets.US_ASCII));
+    this.checkResponse = Unpooled.wrappedBuffer(checkResponse.getBytes(StandardCharsets.US_ASCII));
+  }
+
+  @Override
+  public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
+    ByteBuf buf = (ByteBuf) msg;
+    if (buf.equals(checkRequest)) {
+      ChannelFuture unusedFuture = ctx.writeAndFlush(checkResponse);
+    }
+    buf.release();
+  }
+}