Settings: Make APN of CT can be edited in default mode

The APN of CT can't be edited, because that the read_only property
is true.

In order to make APN of CT can be edited, the rules of editing APN
is changed.

Change-Id: I299c8d18a3a9e21c6896bc8c7b41b3b15ad7c8ef
CRs-Fixed: 1066739