Add product_config module

product_config module will export productVariables struct (defined in
android/variable.go). Other modules can depend on product_Config module
to read product variables, without needing to update Soong.

Bug: 346214955
Test: build and see output of product_config module
Change-Id: I06e1d2716dc2a8b90984bb46c5fc901cd1ca5020

Android.bp

@@ -136,3 +136,8 @@ buildinfo_prop {
 all_apex_contributions {
     name: "all_apex_contributions",
 }
+
+product_config {
+    name: "product_config",
+    visibility: ["//visibility:private"],
+}