apns: Remove user_editable="false" for user visible APNs

This also removes user_visible="false" for some AT&T WAP APNs as
the rest of the AT&T MVNOs APNs had it visible but these ones
weren't.

The reasoning for this is that we can't (and no one can ever)
guarantee a single static APN list is correct for all devices
all the time. Let's not leave users in a situation where they
can't attempt to recover themselves by fixing their APN without
jumping through hoops to copy the APN fields into a new APN.

TODO: Evaluate how user_visible should be used.

Change-Id: I2b4975db9d52943390b61b50b9258746eab447c9