Make Config.ProductVariables private

All access to these should be going through the methods on Config /
DeviceConfig.

Bug: 76168832
Test: m blueprint_tools
Change-Id: I47512dd58fb1a1a3f25838a9b1adaed2c41af8d3

android/arch.go

@@ -821,7 +821,7 @@ func forEachInterface(v reflect.Value, f func(reflect.Value)) {
 
 // Convert the arch product variables into a list of targets for each os class structs
 func decodeTargetProductVariables(config *config) (map[OsClass][]Target, error) {
-	variables := config.ProductVariables
+	variables := config.productVariables
 
 	targets := make(map[OsClass][]Target)
 	var targetErr error