Bug 1033207 - Do not override the custom apn settings

https://bugzilla.mozilla.org/show_bug.cgi?id=1033207
Could you leave a message informing here about we want to keep the custom APNs with this change please? You know I'm really bad at naming but 'mergeDefaultApnSettings()' seems confusing. Something like 'mergeAndKeepCustomApnSettings()' seems better.