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/paths.go b/android/paths.go
index dc89bfd..87efe53 100644
--- a/android/paths.go
+++ b/android/paths.go
@@ -719,14 +719,14 @@
 func (p DistPath) writablePath() {}
 
 func (p DistPath) Valid() bool {
-	return p.config.ProductVariables.DistDir != nil && *p.config.ProductVariables.DistDir != ""
+	return p.config.productVariables.DistDir != nil && *p.config.productVariables.DistDir != ""
 }
 
 func (p DistPath) String() string {
 	if !p.Valid() {
 		panic("Requesting an invalid path")
 	}
-	return filepath.Join(*p.config.ProductVariables.DistDir, p.path)
+	return filepath.Join(*p.config.productVariables.DistDir, p.path)
 }
 
 func (p DistPath) RelPathString() string {