core: Resolve address of GRPC_PROXY_EXP hostname

Since this address is never resolved gRPC fails to connect with an
UnresolvedAddressException when the env variable is specified. This
should resolve the address before we attempt to proxy to it.

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

@@ -184,8 +184,7 @@ class ProxyDetectorImpl implements ProxyDetector {
             + "be removed in a future release. Use the JVM flags "
             + "\"-Dhttps.proxyHost=HOST -Dhttps.proxyPort=PORT\" to set the https proxy for "
             + "this JVM.");
-    // Return an unresolved InetSocketAddress to avoid DNS lookup
-    return InetSocketAddress.createUnresolved(parts[0], port);
+    return new InetSocketAddress(parts[0], port);
   }
 
   /**