Avoid adding FORWARD_LOCK flag to an updated system app

Consider the following case:
1. An app exists in /system/app or /vendor/app. (i.e, it's a system app)
2. If the app would be updated as a forward-lock app by downloading from
   market(google play), then it would be installed in /data/app-private.
3. Moreover, if it would be updated by the whole system image (i.e, FOTA
   or something like that), then the app would be applied a FORWARD_LOCK
   flag.
The app SHOULD NOT have a FORWARD_LCOK flag. Because the resource path
of such app does not refer to the proper resources to access.