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

android/variable.go

@@ -522,6 +522,8 @@ type ProductVariables struct {
 	OemProperties []string `json:",omitempty"`
 
 	ArtTargetIncludeDebugBuild *bool `json:",omitempty"`
+
+	SystemPropFiles []string `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {