Allow OEMs to restrict access to secondary display

This is crucial for automotive use-cases when OEMs want to restrict
access to instrument cluster display by third-party apps

Bug: 28864915
Bug: 70840956
Test: manually verify that secondary display with private flag is not
accessible by third-party apps and accessible to apps with system uid

Change-Id: I39e36c2ea73d879a4c7b01220ae852298d868fd5