priv-app

https://android.googlesource.com/platform/frameworks/base/+/ccbf84f
http://projectmaxs.org/documentation/systemapp.html
http://stackoverflow.com/questions/19868138/aosp-privileged-vs-system-app

...and only apps whose APKs are in that directory are allowed
to use signatureOrSystem permissions without sharing the platform cert.

[Note]  /system/priv-app, it will have platform privilege (to use platform api)