When retrying disassociate an ApnContext from a DC only when failed or idle.

Disassociating, is setting an ApnContext DataConnection to null. When its
null the DataConnectionTracker will not find the DataConnection when the
next RIL_UNSOL_DATA_CALL_LIST_CHANGED arrives and it sends a notification
that the connection has been disconnected. When the notification is
received by the ConnectivityService it will reset the connection and now
there will truly be no connection.

Typically, when the DataConnectionTracker sees that the default connection
is been removed, it will automatically retry the connection. But this won't
happen in this case, because there was no error to cause the retry.

Bug: 6884995
Bug: 7343098
Change-Id: Ia36316151bc5ae9a0ef9b48b1841816198862127