Support passing flag parameters to M4

This will be used to guard sepolicy changes. Also this adds default
modules for se_policy_conf and contexts modules.

Bug: 306563735
Test: build
Change-Id: I9b3460aaca07d325e0f83a1e2bf0e57caa498101
diff --git a/build/soong/flags.go b/build/soong/flags.go
new file mode 100644
index 0000000..b1aebac
--- /dev/null
+++ b/build/soong/flags.go
@@ -0,0 +1,54 @@
+// Copyright (C) 2023 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 selinux
+
+import (
+	"android/soong/android"
+)
+
+type flagsProperties struct {
+	// List of flags to be passed to M4 macro.
+	Flags []string
+}
+
+type flaggableModule interface {
+	android.Module
+	flagModuleBase() *flaggableModuleBase
+	getBuildFlags(ctx android.ModuleContext) map[string]string
+}
+
+type flaggableModuleBase struct {
+	properties flagsProperties
+}
+
+func initFlaggableModule(m flaggableModule) {
+	base := m.flagModuleBase()
+	m.AddProperties(&base.properties)
+}
+
+func (f *flaggableModuleBase) flagModuleBase() *flaggableModuleBase {
+	return f
+}
+
+// getBuildFlags returns a map from flag names to flag values.
+func (f *flaggableModuleBase) getBuildFlags(ctx android.ModuleContext) map[string]string {
+	ret := make(map[string]string)
+	for _, flag := range android.SortedUniqueStrings(f.properties.Flags) {
+		if val, ok := ctx.Config().GetBuildFlag(flag); ok {
+			ret[flag] = val
+		}
+	}
+	return ret
+}