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
diff --git a/Android.mk b/Android.mk
index 9ebe603..e487214 100644
--- a/Android.mk
+++ b/Android.mk
@@ -188,7 +188,7 @@
 
 # Builds paths for all policy files found in BOARD_VENDOR_SEPOLICY_DIRS.
 # $(1): the set of policy name paths to build
-build_vendor_policy = $(call build_policy, $(1), $(PLAT_VENDOR_POLICY) $(BOARD_VENDOR_SEPOLICY_DIRS))
+build_vendor_policy = $(call build_policy, $(1), $(BOARD_PLAT_VENDOR_POLICY) $(BOARD_VENDOR_SEPOLICY_DIRS))
 
 # Builds paths for all policy files found in BOARD_ODM_SEPOLICY_DIRS.
 build_odm_policy = $(call build_policy, $(1), $(BOARD_ODM_SEPOLICY_DIRS))
@@ -1230,7 +1230,7 @@
 
 include $(BUILD_SYSTEM)/base_rules.mk
 
-vnd_svcfiles := $(call build_policy, vndservice_contexts, $(PLAT_VENDOR_POLICY) $(BOARD_VENDOR_SEPOLICY_DIRS) $(REQD_MASK_POLICY))
+vnd_svcfiles := $(call build_policy, vndservice_contexts, $(BOARD_PLAT_VENDOR_POLICY) $(BOARD_VENDOR_SEPOLICY_DIRS) $(BOARD_REQD_MASK_POLICY))
 
 vndservice_contexts.tmp := $(intermediates)/vndservice_contexts.tmp
 $(vndservice_contexts.tmp): PRIVATE_SVC_FILES := $(vnd_svcfiles)