Now newer system policy and older vendor policy can be built together by
setting following variables:
- BOARD_SEPOLICY_VERS
- BOARD_REQD_MASK_POLICY (copy of older system/sepolicy/reqd_mask)
- BOARD_PLAT_VENDOR_POLICY (copy of older system/sepolicy/vendor)
- BOARD_(SYSTEM_EXT|PRODUCT)_(PUBLIC|PRIVATE)_PREBUILT_DIRS (copy of
older system_ext and product policies)
Bug: 168159977
Test: try normal build and mixed build
Test: boot and check selinux denials
Change-Id: Ie239580433ffd293fa6891cd665fb5ef83c0a14f