Making clients pass the raw GSM PDU value isn't very useful
since it's not exactly trivial to create programmatically.
Instead, let's make validity be units of 5 minutes and MM
will automatically figure out how to map that to the PDU
value.
There's no encoding information about 8-bit SMS messages,
and they are often binary things like ringtones or voicemail
indicator commands. Since there's no point to our parsing
them just let clients deal with it.