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/Android.bp b/build/soong/Android.bp
index 83b31b4..0abfdf6 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -35,6 +35,7 @@
         "build_files.go",
         "cil_compat_map.go",
         "compat_cil.go",
+        "flags.go",
         "mac_permissions.go",
         "policy.go",
         "selinux.go",