Move split file_contexts to /system and /vendor

Build file_contexts.bin on legacy builds.
Test: Marlin and Bullhead build and boot with no new denials.
Test: Marlin and Bullhead recovery boots with no new denials.
Test: Bullhead boots with file_contexts.bin in /
Test: Marlin boot with /system/etc/selinux/plat_file_contexts and
      /vendor/etc/selinux/nonplat_file_contexts.
Bug: 36002414

Change-Id: Ide8498b3c86234d2f93bb22a7514d132c33067d6