Dont delete .flat files

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Utilities/Files.cs

@@ -346,6 +346,7 @@ public static bool ExtractAll (ZipArchive zip, string destination, Action<int, i
 				foreach (var file in Directory.GetFiles (destination, "*", SearchOption.AllDirectories)) {
 					var outfile = Path.GetFullPath (file);
 					if (outfile.Contains ("/__MACOSX/") ||
+							outfile.EndsWith (".flat", StringComparison.OrdinalIgnoreCase) ||
 							outfile.EndsWith ("__AndroidLibraryProjects__.zip", StringComparison.OrdinalIgnoreCase) ||
 							outfile.EndsWith ("/__MACOSX", StringComparison.OrdinalIgnoreCase) ||
 							outfile.EndsWith ("/.DS_Store", StringComparison.OrdinalIgnoreCase))