Add sign_sepolicy_path for a binary to sign sepolicy.apex.

Bug: b/217570541
Test: sign_apex --container_key=testdata/testkey
--payload_key=testdata/testkey_RSA4096.key
--codename_to_api_level_map=S:31,T:32,UpsideDownCake:33
-e SEPolicy-33.zip=testdata/testkey com.android.sepolicy.apex
signed_sepolicy.apex

Change-Id: Ifdd1c6e7db07cbc4674450ab0e2226494c98605a