Add a workaround for a platform JAR parsing bug.

The java.util.jar implementation through Android 1.6 has a
bug where if the signature file in META-INF is a multiple
of 1024 bytes, it will throw an IOException attempting to
read it.

If signapk would produce a CERT.SF in a multiple of 1024
bytes, add an extra CRLF to the end of the file.

Bug: 3019677
Change-Id: I23d4a36e12e224be600d3ac39379b5b5a022a628