Restore write_delete_file

(cherry picked from commit 5baa164aec06a9973a68303bc7752aa76e5c68e5)

Change-Id: Ifa9a157346edd479ce03982baf435ab213dc2e79
diff --git a/libs/androidfw/BackupHelpers.cpp b/libs/androidfw/BackupHelpers.cpp
index 4dcb6e7..227de3b 100644
--- a/libs/androidfw/BackupHelpers.cpp
+++ b/libs/androidfw/BackupHelpers.cpp
@@ -202,6 +202,13 @@
 }
 
 static int
+write_delete_file(BackupDataWriter* dataStream, const String8& key)
+{
+    LOGP("write_delete_file %s\n", key.string());
+    return dataStream->WriteEntityHeader(key, -1);
+}
+
+static int
 write_update_file(BackupDataWriter* dataStream, int fd, int mode, const String8& key,
         char const* realFilename)
 {