Make priv apps not trust user added CAs by default

Privileged applications provide core system functionality and as such a
MiTM in one can put the entire system at risk. These applications should
not be trusting user added CAs by default.

Bug: 65406503
Test: runtest --path framework/base/tests/NetworkSecurityConfigTest
Change-Id: I033258fe1c66ad245d172899df52e9cd02e9ca75