Do not allow privacy guard for core system apps.

Hide any applications that have the system UID as well as System UI from
privacy guard. Enabling this on these core apps can have disastrous
consequences, since many privacy guard authorization requests will queue
up. This can cause the system process or System UI to be killed when
they ANR while they wait at the end of a very long queue for the user to authorize them.

By using privacy guard, you are trusting that the system will control
and protect these permissions in the first place; there is no need to protect
the system from itself.

Change-Id: I478d6a6783a4c06fa7ad01a96c413290b232636c