Fix vendor contexts files in mixed build

BOARD_PLAT_VENDOR_POLICY should be used for all vendor stuff, when in
mixed sepolicy build (BOARD_SEPOLICY_VERS != PLATFORM_SEPOLICY_VERSION).
This fixes an issue that system/sepolicy/vendor has been incorrectly
used in mixed sepolicy build.

Bug: 205924657
Test: Try AOSP + rvc-dev mixed sepolicy build
1) copy cuttlefish sepolicy prebuilts from rvc-dev branch.
2) set prebuilt variables:
  - BOARD_PLAT_VENDOR_POLICY
  - BOARD_REQD_MASK_POLICY
  - BOARD_(SYSTEM_EXT|PRODUCT)_PRIVATE_PREBUILT_DIRS
  - BOARD_SEPOLICY_VERS
3) lunch aosp_cf_x86_64_phone-userdebug; m selinux_policy
4) compare $OUT/vendor/etc/selinux with rvc-dev's artifacts.

Change-Id: I2ed1e25255c825c24dab99ae4903328b0400c414
5 files changed