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: Ic7c4a53d0df0cccd45eb699e236a92c8c0bc2d56

android/variable.go

@@ -486,6 +486,8 @@ type ProductVariables struct {
 	ReleaseAconfigFlagDefaultPermission string `json:",omitempty"`
 
 	KeepVndk *bool `json:",omitempty"`
+
+	CheckVendorSeappViolations *bool `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {