Build split file_contexts on TREBLE builds

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: I66f138fc3ad808df0480e0467cee03fd40177f31