GsmCdmaPhone: Return dummy ICCID serial for NV sub

In case of NV sub there's no ICC, so both getIccSerialNumber()
and getFullIccSerialNumber() return null.

Since getFullIccSerialNumber() is used to set/get the ID of
each PhoneAccountHandle, devices with NV subs can't register
proper phone accounts other than the emergency one.

getIccSerialNumber() is instead used by CarrierConfigLoader
to store and load configs.

Fix both the problems forcing dummy serials for NV subscriptions.

Change-Id: I610115e9d9f5b3b86b36fddcf13dbed6387e377c