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.

This change was previously reverted because it affected the location
of property_contexts in recovery. Now that we have separate tagets for
recovery (see ec78c377), this change
no longer affects is recovery.

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

Test: cts-tradefed run singleCommand cts --skip-device-info \
      --skip-preconditions --skip-connectivity-check \
      --abi arm64-v8a --module CtsSecurityHostTestCases \
      -t android.security.cts.SELinuxHostTest#testAospPropertyContexts

      This test was performed on bullhead (non A/B device) and sailfish
      (A/B device).

Test: Clean build, flash, device boots with no additional denials.
      Rebooting to recovery, recovery boots fine with no denials.
      This test was performed on bullhead (non A/B device) and sailfish
      (A/B device).
Bug: 36002573

(cherry picked from commit 4cb628a3)

Change-Id: I0b145c58669fb31bc39d57f36eef1190425a8328