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/Android.bp b/Android.bp
index ed766e4..999333d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -824,6 +824,12 @@
     product_specific: true,
 }
 
+sepolicy_vers {
+    name: "plat_sepolicy_vers.txt",
+    version: "vendor",
+    vendor: true,
+}
+
 //////////////////////////////////
 // SELinux policy embedded into CTS.
 // CTS checks neverallow rules of this policy against the policy of the device under test.
@@ -888,3 +894,10 @@
     filter_out: [":microdroid_plat_pub_versioned.cil"],
     installable: false,
 }
+
+sepolicy_vers {
+    name: "microdroid_plat_sepolicy_vers.txt",
+    version: "platform",
+    stem: "plat_sepolicy_vers.txt",
+    installable: false,
+}