Add se_policy_conf module to build policy.conf

This adds a new soong module that transforms selinux policy files to
policy.conf file. It uses m4 macro with various variables, and replaces
transform-policy-to-conf macro in system/sepolicy/definitions.mk.

The module will be used when building:
- policy cil files shipped to the device
- CTS tests that needs general_policy.conf

Bug: 33691272
Test: try building se_policy_conf with se_build_files
Change-Id: Ie1082a8193c2205992b425509b9d5bfa4b495b2f
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index 54173e0..4e1d27a 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -34,6 +34,7 @@
         "build_files.go",
         "cil_compat_map.go",
         "filegroup.go",
+        "policy.go",
         "selinux.go",
         "selinux_contexts.go",
     ],