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/config.go b/android/config.go
index a9833da..3af63d0 100644
--- a/android/config.go
+++ b/android/config.go
@@ -946,6 +946,10 @@
 	return String(c.config.productVariables.Platform_vndk_version)
 }
 
+func (c *deviceConfig) ProductVndkVersion() string {
+	return String(c.config.productVariables.ProductVndkVersion)
+}
+
 func (c *deviceConfig) ExtraVndkVersions() []string {
 	return c.config.productVariables.ExtraVndkVersions
 }