Exempting privileged apps from permission check in package installer

If a privileged app does want to be treated as a trusted source, it can
pass in the extra EXTRA_NOT_UNKNOWN_SOURCE and does not need to declare
the permission REQUEST_INSTALL_PACKAGES. In such a case it will not be
shown in the list under settings to the user.

Test: Manually tested that sync auth successfully installs dpc during
profile provisioning flow

Bug: 37623444
Change-Id: I70083980f3b8e43de78d2f1a781d55e19fdc00ee