Update install-permission logic to use new FLAG_PRIVILEGED

We no longer grant all "signatureOrSystem" type permissions to all apps
bundled on the system partition; there is a build-time grant of
privileged status.  The logic for granting install permission now
checks the caller's privileged status, not just its apk location.

Bug 8765951

Change-Id: Ib88f4b0911743bd6bfd3458302fe88518e08ac86