Rename product_services to system_ext

Bug: 134359158
Test: build and check if system_ext.img is created
Change-Id: Ice3229baf97a94c24a6eea22e7d4571941d7f843
diff --git a/android/config.go b/android/config.go
index e4012fa..c0ed50f 100644
--- a/android/config.go
+++ b/android/config.go
@@ -901,11 +901,11 @@
 	return "product"
 }
 
-func (c *deviceConfig) ProductServicesPath() string {
-	if c.config.productVariables.ProductServicesPath != nil {
-		return *c.config.productVariables.ProductServicesPath
+func (c *deviceConfig) SystemExtPath() string {
+	if c.config.productVariables.SystemExtPath != nil {
+		return *c.config.productVariables.SystemExtPath
 	}
-	return "product_services"
+	return "system_ext"
 }
 
 func (c *deviceConfig) BtConfigIncludeDir() string {