Add odm sepolicy support to libselinux

Also renames nonplat_* to vendor_*.

We declare three arrays for each type of context file, and select at
most one file for each array.
    - {type}_plat[]
    - {type}_vendor[]
    - {type}_odm[]

With this, it's possible to support backward compatibility.
    - treble / pre-treble (files under root or system/vendor/odm)
    - nonplat / vendor (old and new filenames)
    - odm partition exists or not

Bug: 64240127
Test: boot sailfish normally without odm
Test: boot another device having odm
Change-Id: I9963118f8d4c87d0a520ec3d10db7a3e8411df53
2 files changed