Allow library spec and app configs to be read from odm partition

Currently only feature specs are allowed to be read from /oem/etc/permissions.
ODM may want to add their own library specs and app configs (ie, "app-link",
"system-user-whitelisted-app" and "system-user-blacklisted-app") but they can
only add them to /system/etc/permissions and thus polluted the system.img.
With the fine-grained permission flag introduced in this change, we also tighten
the permission for OEMs so they can only customize the feature specs.

Bug: 25759900
Change-Id: I44ef1380b160c2b6a53f2af6de0c5f5eae27a859