Correct location of property_contexts for TREBLE devices

This makes the build system, for TREBLE devices only, place
plat_property_contexts under /system/etc/selinux and
nonplat_property_contexts under /vendor/etc/selinux. For other devices
these files are placed under /, same as before.

Test: *_property_contexts in correct locations when
      PRODUCT_FULL_TREBLE is set to true and when it is set to false.
Bug: 36002573

Change-Id: I7e30e64918bb3ee671fa8c7a2e30ed96a9cc1ad7