Settings: Make APN of CT can be edited and be saved.

The APN of CT can't be edited, because that the read_only property
is true.
The APN of CT can't be actually modified because it don't successfullly
be saved.

Change-Id: I4a14deca29c9951eb105304fcd2d0a082701f5d5
CRs-Fixed: 1066739