System apps location on Android Jelly Bean
To my knowledge, before Kitkat .apk files of system apps all went to /system/app. Kitkat added the /system/priv-app directory. The structure having an additional directory level came later (LP or MM).

References:


AOSP Privileged vs System app
What's the difference of /system/app/ and /system/priv-app
Android 5.0/Lollipop: Force rescan of /system/priv-app