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 and boot a device
Test: checks the content of $OUT/vendor/etc/selinux/vendor_sepolicy.cil
      is the same as before
Change-Id: I0b64f1088f413172e97b579b4f7799fa392762df