beckham: Use PRODUCT_*_SEPOLICY_DIRS to build product sepolicy.

We are restoring the old behavior of BOARD_PLAT_*_SEPOLICY_DIR that
extends system sepolicy. To that end, we use another make variable to
specify product sepolicy.

Bug: 119305624
Test: m selinux_policy
Change-Id: I67636b7aaff2bd5ae5309981f9fa2f8ce51edf6f

BoardConfig.mk

@@ -54,8 +54,8 @@ TARGET_HAS_NO_WLAN_STATS := true
 ENABLE_VENDOR_RIL_SERVICE := true
 
 # SELinux
-BOARD_PLAT_PRIVATE_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy-mods/private
-BOARD_PLAT_PUBLIC_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy-mods/public
+PRODUCT_PRIVATE_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy-mods/private
+PRODUCT_PUBLIC_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy-mods/public
 BOARD_VENDOR_SEPOLICY_DIRS += $(DEVICE_PATH)/sepolicy-mods/vendor
 
 # inherit from the proprietary version