Prefer seapp/service_contexts from /system & /vendor

This changes libselinux to prefer loading seapp_contexts and
service_contexts from /system/etc/selinux & /vendor/etc/selinux,
while falling back to the previous behavior of loading from /.

Test: Build and boot Marlin and Bullhead.
Test: Contexts split between /system and /vendor on Marlin.
      Remains stored in / on Bullhead.
Bug: 36002816
Bug: 36002427

Change-Id: I912ffc765c3c9f1610fc641a0bc2b1494f809918
1 file changed