Use OnceKey in art.go

Once is changing to take a type-safe key, use android.NewOnceKey
in art.go.

Test: m checkbuild
diff --git a/build/art.go b/build/art.go
index 5236e31..2ec1917 100644
--- a/build/art.go
+++ b/build/art.go
@@ -253,8 +253,10 @@
 	ctx.AppendProperties(p)
 }
 
+var testMapKey = android.NewOnceKey("artTests")
+
 func testMap(config android.Config) map[string][]string {
-	return config.Once("artTests", func() interface{} {
+	return config.Once(testMapKey, func() interface{} {
 		return make(map[string][]string)
 	}).(map[string][]string)
 }
@@ -356,7 +358,7 @@
 func libartStaticDefaultsFactory() android.Module {
 	c := &codegenProperties{}
 	module := cc.DefaultsFactory(c)
-	android.AddLoadHook(module, func(ctx android.LoadHookContext) {	codegen(ctx, c, true) })
+	android.AddLoadHook(module, func(ctx android.LoadHookContext) { codegen(ctx, c, true) })
 
 	return module
 }