Export default apex version from updatable_modules.go to Bazel's
apex_toolchain through soong_injection.

This is necessary to keep both Bazel and Soong in sync.

Test: presubmit
Bug: 238723069
Fixes: 238153998

Change-Id: Ibf4a8ed91d358dc7800c10e4112485a52f0606bb
diff --git a/apex/bp2build.go b/apex/bp2build.go
new file mode 100644
index 0000000..221ab13
--- /dev/null
+++ b/apex/bp2build.go
@@ -0,0 +1,30 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package apex
+
+import (
+	"android/soong/android"
+	"strings"
+)
+
+// This file contains the bp2build integration for the apex package.
+
+// Export constants as Starlark using bp2build to Bazel.
+func BazelApexToolchainVars() string {
+	content := []string{
+		"# GENERATED BY SOONG. DO NOT EDIT.",
+		"default_manifest_version = " + android.DefaultUpdatableModuleVersion, // constants.go is different in every branch.
+	}
+	return strings.Join(content, "\n")
+}