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

ui/build/soong.go

@@ -694,6 +694,7 @@ func runSoong(ctx Context, config Config) {
 		}
 	}
 	distFile(ctx, config, config.SoongVarsFile(), "soong")
+	distFile(ctx, config, config.SoongExtraVarsFile(), "soong")
 
 	if !config.SkipKati() {
 		distGzipFile(ctx, config, config.SoongAndroidMk(), "soong")