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/config.go

@@ -2135,3 +2135,7 @@ func (c *config) UseDebugArt() bool {
 
 	return Bool(c.productVariables.Eng)
 }
+
+func (c *config) SystemPropFiles(ctx PathContext) Paths {
+	return PathsForSource(ctx, c.productVariables.SystemPropFiles)
+}