Only maintain maps between current and previous selinux versions.

New maintenance scheme for mapping files:
Say, V is the current SELinux platform version, then at any point in time we
only maintain (V->V-1) mapping. (V->V-n) map is constructed from top (V->V-n+1)
and bottom (V-n+1->V-n) without changes to previously maintained mapping files.

Caveats:
- 26.0.cil doesn't technically represent 27.0->26.0 map, but rather
current->26.0. We'll fully migrate to the scheme with future releases.

Bug: 67510052
Test: adding new public type only requires changing the latest compat map
Change-Id: Iab5564e887ef2c8004cb493505dd56c6220c61f8
diff --git a/tests/Android.bp b/tests/Android.bp
index abb5e35..670d29d 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -63,3 +63,11 @@
     required: ["libsepolwrap"],
     defaults: ["py2_only"],
 }
+
+python_binary_host {
+    name: "combine_maps",
+    srcs: [
+        "combine_maps.py",
+        "mini_parser.py",
+    ],
+}