Add a skeleton sepolicy compat generator

sepolicy_generate_compat will be used to generate compat files for ToT,
based on the mapping file from aosp_arm64-userdebug target of {ver}
source tree. For now, it only supports downloading a mapping file
system/etc/selinux/mapping/{ver}.cil from the Android build server.

Bug: 214336258
Test: sepolicy_generate_compat --branch sc-v2-dev --version 32.0
Change-Id: I48043c71a6866aa385ecd67462f7678561cc5a38
diff --git a/tools/Android.bp b/tools/Android.bp
index c480dc2..1ec129d 100644
--- a/tools/Android.bp
+++ b/tools/Android.bp
@@ -63,3 +63,8 @@
     name: "insertkeys",
     srcs: ["insertkeys.py"],
 }
+
+python_binary_host {
+    name: "sepolicy_generate_compat",
+    srcs: ["sepolicy_generate_compat.py"],
+}