Fixed handover issues

1. If the reported network is unknown, ignore it.
2. If receiving handover requests back to back, gracefully
   ignore the 2nd one without changing the transport type
   immediately. Transport manager now will be notified the
   handover result.
3. Preserved the network agent before issuing handover request.
   This prevents some races that source data connection
   disappeared before handover is completed.
4. If no live data connection but there are already networks
   there, directly move the network requests to the new transport.
5. Do not notify connectivity service data disconnected if the data
   is being handovered to the new transport.
6. For auto attach, it should be only for cellular.
7. Retry handover setup when failed.
8. Immediately remove the network request from the source transport
   once handover is completed, whether succeeded or not. The reason
   is that qualified network service has reported the current transport.
   So even setup is failed, we should still stick on that transport.
   The setup can be retried later.

Test: Telephony sanity tests
Bug: 130366327
Bug: 130312660
Bug: 130538538
Bug: 130743820

Change-Id: Ie927bbfd7972b992a33abec7a3899d95a74b4f49