Allow system app to take ownership of built-in permissions

Previous commit bfed9f34 makes the preloaded system app take
precedence over third-party apps when defining permissions, but
it also makes it be able to override android built-in permissions.

Now allow preloaded system app to take the ownership of built-in
permissions instead of overriding it.

Change-Id: I10d588d0284e4316ea4be552fd6191f33e3c725b