Add TARGET_SYSTEM_PROP to system build.prop

System partition's build_prop output now contains TARGET_SYSTEM_PROP.
Soong needs to know paths to add them to ninja build dependency graph.

Bug: 322090587
Test: build and compare both build.prop files
Change-Id: Ide73e876d28eda0cced2ec08bd3c680115025ad8
diff --git a/android/variable.go b/android/variable.go
index b2173ec..ec4d0ea 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -522,6 +522,8 @@
 	OemProperties []string `json:",omitempty"`
 
 	ArtTargetIncludeDebugBuild *bool `json:",omitempty"`
+
+	SystemPropFiles []string `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {