Only report "unknown metadata" once per restore helper

Also removes the auto-free object, replacing it with direct memory manipulation.
diff --git a/include/utils/BackupHelpers.h b/include/utils/BackupHelpers.h
index 759a0cc..b1f5045 100644
--- a/include/utils/BackupHelpers.h
+++ b/include/utils/BackupHelpers.h
@@ -137,6 +137,7 @@
 
 private:
     void* m_buf;
+    bool m_loggedUnknownMetadata;
     KeyedVector<String8,FileRec> m_files;
 };