Settings: fix force close when resetting network settings

Create handler inside ThreadAsyncTask, but ThreadAsyncTask does not have
looper, so cannot dispatch message and results in force close.

Encapsulated restore default apn processing into ThreadAsyncTask, now does not
need handler yet. So can remove them.

Change-Id: I28a1dccc30e2c89f649e468940d3f369b10b7269
CRs-Fixed: 2542059