Add se_build_files module

se_build_files module globs given srcs from sepolicy directories and
acts as a filegroup with the following tags, which can be used to build
system side policy files.

- plat
- plat_public
- system_ext
- system_ext_public
- product
- product_public
- reqd_mask

se_build_files module acts like the build_policy macro in Android.mk.
Normal genrule module can't easily handle that, because both file order
and directory order matter.

Support for vendor/odm is to be added in the future.

Bug: 33691272
Test: inspect se_build_files with above tags and compare it to ninja
Change-Id: Id7c57b01c78fc14ac5e8eeeb074a6fc21d271e84
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index 5f951ce..54173e0 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -31,6 +31,7 @@
         "soong-sysprop",
     ],
     srcs: [
+        "build_files.go",
         "cil_compat_map.go",
         "filegroup.go",
         "selinux.go",