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
diff --git a/android/variable.go b/android/variable.go
index db875ff..9c87614 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -272,7 +272,7 @@
 		property := "product_variables." + proptools.PropertyNameForField(name)
 
 		// Check that the variable was set for the product
-		val := reflect.ValueOf(mctx.Config().ProductVariables).FieldByName(name)
+		val := reflect.ValueOf(mctx.Config().productVariables).FieldByName(name)
 		if !val.IsValid() || val.Kind() != reflect.Ptr || val.IsNil() {
 			continue
 		}