Export make vars using MakeVars method

ed023eca73 introduced a new (better) way
of exporting make vars from singletone. apex_keys_text singletone is
switched to the new method.

Test: inspect out/soong/make_vars-<target>.mk
Check SOONG_SOONG_APEX_KEYS_FILE is set

Change-Id: Ia218852ba9ae40070cb6c99340d97e0c77d19841

apex/key.go

@@ -29,7 +29,6 @@ var String = proptools.String
 func init() {
 	android.RegisterModuleType("apex_key", apexKeyFactory)
 	android.RegisterSingletonType("apex_keys_text", apexKeysTextFactory)
-	android.RegisterMakeVarsProvider(pctx, apexKeysFileProvider)
 }
 
 type apexKey struct {
@@ -108,11 +107,12 @@ func (m *apexKey) AndroidMk() android.AndroidMkData {
 
 ////////////////////////////////////////////////////////////////////////
 // apex_keys_text
-type apexKeysText struct{}
+type apexKeysText struct {
+	output android.OutputPath
+}
 
 func (s *apexKeysText) GenerateBuildActions(ctx android.SingletonContext) {
-	output := android.PathForOutput(ctx, "apexkeys.txt")
-	*apexKeysFile(ctx.Config()) = output.String()
+	s.output = android.PathForOutput(ctx, "apexkeys.txt")
 	var filecontent strings.Builder
 	ctx.VisitAllModules(func(module android.Module) {
 		if m, ok := module.(android.Module); ok && !m.Enabled() {
@@ -131,27 +131,18 @@ func (s *apexKeysText) GenerateBuildActions(ctx android.SingletonContext) {
 	})
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        android.WriteFile,
-		Description: "apex_keys.txt",
-		Output:      output,
+		Description: "apexkeys.txt",
+		Output:      s.output,
 		Args: map[string]string{
 			"content": filecontent.String(),
 		},
 	})
 }
 
-var apexKeysFileKey = android.NewOnceKey("apexKeysFile")
-
-func apexKeysFile(config android.Config) *string {
-	return config.Once(apexKeysFileKey, func() interface{} {
-		str := ""
-		return &str
-	}).(*string)
-}
-
 func apexKeysTextFactory() android.Singleton {
 	return &apexKeysText{}
 }
 
-func apexKeysFileProvider(ctx android.MakeVarsContext) {
-	ctx.Strict("SOONG_APEX_KEYS_FILE", *apexKeysFile(ctx.Config()))
+func (s *apexKeysText) MakeVars(ctx android.MakeVarsContext) {
+	ctx.Strict("SOONG_APEX_KEYS_FILE", s.output.String())
 }