Move the Auto Generated text to the StringBuilder

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Link.cs

@@ -122,8 +122,8 @@ public async override System.Threading.Tasks.Task RunTaskAsync ()
 				if (!string.IsNullOrEmpty (ProguardRuleOutput)) {
 					// combine the "proguard" temp files into one file.
 					var sb = new StringBuilder ();
+					sb.AppendLine ("#Auto Generated file. Do not Edit.");
 					HashSet<string> output = new HashSet<string> ();
-					output.Add ("#Auto Generated file. Do not Edit.");
 					lock (rulesFiles) {
 						foreach (var file in rulesFiles) {
 							var lines = File.ReadAllLines (file);