Using a python script to build sepolicy

Current sepolicy CIL files are built by several command-line tools
in Android.mk. This change extracts some of the build logic into a
python script to relief the effort in Android.mk.

The first command is `build_sepolicy build_cil`. It's possible to add
more sub-commands under the build_sepolicy script in the future.

Bug: 64240127
Test: build bullhead/taimen
Change-Id: Ie0ae4fc5256a550c72954cde5d5dd213a22d159a