PackageManager: Restrict system packages to protected storage

Don't allow packages signed with the platform key to be
installed into user (or app) controlled storage

Change-Id: I7390b3bdf5289411f7a335c69635bed9f74bbfb3

PackageManager: Limit the platform-key protection to test-keys builds

This block (change I7390b3bdf5289411f7a335c69635bed9f74bbfb3) is intended
to protect the system from abuse of well-known keys. That's not the case
if the system is signed with dev or release keys.

Change-Id: Ifb66f775dd28c577447634ae177442732a18b592