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/makevars.go b/android/makevars.go
index 3792357..3094a48 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -111,7 +111,7 @@
 		return
 	}
 
-	outFile := PathForOutput(ctx, "make_vars"+proptools.String(ctx.Config().ProductVariables.Make_suffix)+".mk").String()
+	outFile := PathForOutput(ctx, "make_vars"+proptools.String(ctx.Config().productVariables.Make_suffix)+".mk").String()
 
 	if ctx.Failed() {
 		return