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/Android.bp

@@ -83,6 +83,7 @@ bootstrap_go_package {
         "plugin.go",
         "prebuilt.go",
         "prebuilt_build_tool.go",
+        "product_config.go",
         "proto.go",
         "provider.go",
         "raw_files.go",