Make package installer a privileged core app

Package installer should be a core app, so it is available when we boot
in a core app only mode for the user to enter a pattern to decrypt.

Package installer is now a privileged app as it needs provileged
permissions in the additions to the ones we explicitly grant it.
This is to ensure it will get these permissions when its cert changes.

bug:22341030

Change-Id: I52137fe181b84fca6d49fb6b1b363527f0fd323c