apns-conf.xml: Add SIMPLE Mobile APN

I noticed that the APN I needed wasn't present, so I added it. It is present in apns-full-conf.xml, albeit under a different name (Tracfone I believe), but not in the config that is actually used (I think).

I got the APN directly from here: http://www.simplemobile.com/wps/portal/home/support/phone-programming

Warm regards,
- Connor Baker