add "EXTERNAL" as special value of LOCAL_CERTIFICATE

Setting LOCAL_CERTIFICATE to "EXTERNAL" now marks an apk (either a
prebuilt or otherwise) as needing the default test key within the
system, but one that should be signed after the target_files is
produced but before sign_target_files_apks does the rest of the
signing.  (We use this to ship apps on the system that are signed by
third parties, like Facebook.)