Add sepolicy_vers for plat_sepolicy_vers.txt

plat_sepolicy_vers.txt stores the version of vendor policy. This change
adds sepolicy_vers module to migrate plat_sepolicy_vers.txt to
Android.bp.

- Device's plat_sepolicy_vers: should be BOARD_SEPOLICY_VERS
- Microdroid's plat_sepolicy_vers: should be PLATFORM_SEPOLICY_VERSION
because all microdroid artifacts are bound to platform

Bug: 33691272
Test: boot device && boot microdroid
Change-Id: Ida293e1cb785b44fa1d01543d52d3f8e15b055c2
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index 6a52fe5..2282112 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -38,6 +38,7 @@
         "policy.go",
         "selinux.go",
         "selinux_contexts.go",
+        "sepolicy_vers.go",
         "versioned_policy.go",
     ],
     pluginFor: ["soong_build"],