Clean up outstanding handover request when tearing down

When tearing down the data connection, we also need to
check if there is any outstanding handover request and
properly respond it. Otherwise TelephonyNetworkFactory
will be stuck waiting there forever for handover complete
messages.

Fix: 187383944
Test: Manual
Change-Id: I202d254a3eda488e853dcc58993a1b91918d4c7d