[Apn] Copy network type into lingering network type

The lingering network type is actually the bit that controls network
torn down. UNKNOWN here means the APN can work on any RAT.

Just copy network type into lingering network type, so network torn down
can work normally.

Fix: 341006308
Test: unit test
Change-Id: Ia60ad3e012bec7364aa35535de1363cd7c0ccdda