Move ProxyResolver to common.

HttpFetcher depends on it and HttpFetcher is in common.

Bug: 28171891
Test: mma
Change-Id: I1783821c4a7d6785ee23df1cde648a3e1ea8fadf
diff --git a/common/http_fetcher.h b/common/http_fetcher.h
index b2fba1c..1f5c945 100644
--- a/common/http_fetcher.h
+++ b/common/http_fetcher.h
@@ -28,7 +28,7 @@
 #include <brillo/message_loops/message_loop.h>
 
 #include "update_engine/common/http_common.h"
-#include "update_engine/proxy_resolver.h"
+#include "update_engine/common/proxy_resolver.h"
 
 // This class is a simple wrapper around an HTTP library (libcurl). We can
 // easily mock out this interface for testing.