Pass privapp specifier for selinux labeling

Allow privileged apps to run in their own priv_app domain.

Motivation:
Untrusted_app is overprivileged due to the inclusion of privileged
apps like gmscore, play store and finsky. Moving these and other
privileged apps to their own domain reduces the permissions required
by untrusted_app.

A separate priv_app domain also protects priv-apps by further
isolating them from third party apps.

Bug: 22033466
Change-Id: I8e6ae5677c5a978301c453d0aa51ebed4459f5a0