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/variable.go b/android/variable.go
index b4f31c6..fcd92d4 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -232,10 +232,10 @@
 	EnableXOM       *bool    `json:",omitempty"`
 	XOMExcludePaths []string `json:",omitempty"`
 
-	VendorPath          *string `json:",omitempty"`
-	OdmPath             *string `json:",omitempty"`
-	ProductPath         *string `json:",omitempty"`
-	ProductServicesPath *string `json:",omitempty"`
+	VendorPath    *string `json:",omitempty"`
+	OdmPath       *string `json:",omitempty"`
+	ProductPath   *string `json:",omitempty"`
+	SystemExtPath *string `json:",omitempty"`
 
 	ClangTidy  *bool   `json:",omitempty"`
 	TidyChecks *string `json:",omitempty"`