Use OutputFilesProvider on aconfig_declarations_group

In the context of incremental soong, the output files
inter-module-communication will be through OutputFilesProvider.
The OutputFileProducer interface will be deprecated.

This CL also removes the unused field "intermediatePath" from
build_flag_declarations and aconfig_declarations

Test: CI
Bug: 339477385
Change-Id: I8417db7ca6ece50b3ecd807fc3b6356aa52b18e0

aconfig/build_flags/declarations.go

@@ -15,7 +15,6 @@
 package build_flags
 
 import (
-	"fmt"
 	"strings"
 
 	"android/soong/android"
@@ -39,8 +38,6 @@ type DeclarationsModule struct {
 		// aconfig files, relative to this Android.bp file
 		Srcs []string `android:"path"`
 	}
-
-	intermediatePath android.WritablePath
 }
 
 func DeclarationsFactory() android.Module {
@@ -53,18 +50,6 @@ func DeclarationsFactory() android.Module {
 	return module
 }
 
-func (module *DeclarationsModule) OutputFiles(tag string) (android.Paths, error) {
-	switch tag {
-	case "":
-		// The default output of this module is the intermediates format, which is
-		// not installable and in a private format that no other rules can handle
-		// correctly.
-		return []android.Path{module.intermediatePath}, nil
-	default:
-		return nil, fmt.Errorf("unsupported build_flags_declarations module reference tag %q", tag)
-	}
-}
-
 func joinAndPrefix(prefix string, values []string) string {
 	var sb strings.Builder
 	for _, v := range values {