Generate product variants by default

PRODUCT_PRODUCT_VNDK_VERSION is set to 'current' by default. Now, we
can generate product variants without checking the
PRODUCT_PRODUCT_VNDK_VERSION build variable. Remove reading the
PRODUCT_PRODUCT_VNDK_VERSION variable from soong and generate product
variants by default.

Bug: 302255959
Test: m
Change-Id: I9a9b2076f4367c5ce9a393bbb206f8dee3884bd8
diff --git a/apex/apex.go b/apex/apex.go
index 090d9c4..f903373 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -736,7 +736,7 @@
 			vndkVersion = deviceConfig.VndkVersion()
 		} else if a.ProductSpecific() {
 			prefix = cc.ProductVariationPrefix
-			vndkVersion = deviceConfig.ProductVndkVersion()
+			vndkVersion = deviceConfig.PlatformVndkVersion()
 		}
 	}
 	if vndkVersion == "current" {