Prevent non-admin users from deleting system apps.

This addresses a security issue where the guest user can remove updates
for system apps.

With this CL, attempts to uninstall/downgrade system apps will fail if
attempted by a non-admin user.

Bug: 170646036
Test: atest DeletePackageHelperTest
Test: manual, try uninstalling system app update as guest
Change-Id: I4e959e296cca9bbdfc8fccc5e5e0e654ca524165