Use board api level for seapp coredomain check

Rather than PRODUCT_SHIPPING_API_LEVEL, use board api level
(BOARD_API_LEVEL or BOARD_SHIPPING_API_LEVEL) to determine whether we
check coredomain violations or not.

Also provides a Makefile variable to override the flag, for targets that
want to turn on the check optionally.

Bug: 280547417
Test: see build command of vendor_seapp_contexts
Change-Id: I177630d33313334ca4a56a9be88b78cff678281e
diff --git a/core/product.mk b/core/product.mk
index b66f1e2..bf85c78 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -430,6 +430,9 @@
 # specified we default to COW version 2 in update_engine for backwards compatibility
 _product_single_value_vars += PRODUCT_VIRTUAL_AB_COW_VERSION
 
+# If set, determines whether the build system checks vendor seapp contexts violations.
+_product_single_value_vars += PRODUCT_CHECK_VENDOR_SEAPP_VIOLATIONS
+
 _product_list_vars += PRODUCT_AFDO_PROFILES
 
 .KATI_READONLY := _product_single_value_vars _product_list_vars