Add a neverallow rule (compile time assertion) that no SELinux domain
other than init can set default_prop. default_prop is assigned to a
property when no more specific label exists for that property.

This ensures that all properties are labeled properly, and that
no-one (other than init) gets access to unknown properties.

Change-Id: If279960f23737e263d4d1b5face7b5c49cda7ae7