Create product variant for product partition

When PRODUCT_PRODUCT_VNDK_VERSION is set to 'current', product
modules are enforced to use only VNDK libs from the system partition
as BOARD_VNDK_VERSION does to vendor partition.

Modules with 'vendor_available: true' create product variant as well
as core and vendor variants. The product variant as an image variant
is used for the modules in /product or /system/product.

It must not affect the current build behavior without
PRODUCT_PRODUCT_VNDK_VERSION set.

Bug: 134099726
Bug: 138966004
Bug: 144534640
Test: build without PRODUCT_PRODUCT_VNDK_VERSION set
Change-Id: I4d3585c110d84493e45bf76d550dc240bb26137f
diff --git a/android/variable.go b/android/variable.go
index 628408e..9647c7a 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -306,6 +306,8 @@
 	ProductPrivateSepolicyDirs []string `json:",omitempty"`
 	ProductCompatibleProperty  *bool    `json:",omitempty"`
 
+	ProductVndkVersion *string `json:",omitempty"`
+
 	TargetFSConfigGen []string `json:",omitempty"`
 
 	MissingUsesLibraries []string `json:",omitempty"`