When reconnecting disassociate dc and apnContext.

This is done in onActionIntentReconnectAlarm, which is called when
retrying after an error, by setting the ApnContext.DataConnection &
DataConnctionAc to null which intern clears the DataConnection.mApnList.

With the mApnList cleared DataConnection.mRefCount will not be incremented
because the DC will not be found by findReadyDataConnection. Thus fixing
at least one cause of 6529189.

Bug: 6529189
Change-Id: I40ff1966f6fbd2f69e97d77b9db7122e4c5b992f