rust: Privatize Cargo* methods on compiler

These methods aren't used outside, and probably shouldn't be.

Bug: 309943184
Test: m nothing
Change-Id: Ia7dd5220ccd10c0136aa16f5e21149eaf7a03ea1

rust/project_json.go

@@ -241,8 +241,8 @@ func (singleton *projectGeneratorSingleton) addCrate(ctx android.SingletonContex
 		ProcMacro:   procMacro,
 	}
 
-	if comp.CargoOutDir().Valid() {
-		crate.Env["OUT_DIR"] = comp.CargoOutDir().String()
+	if comp.cargoOutDir().Valid() {
+		crate.Env["OUT_DIR"] = comp.cargoOutDir().String()
 	}
 
 	for _, feature := range comp.Properties.Features {