TelephonyRegistry: Avoid adding duplicate listener

- Currently, a new callback is created when a listener
  is passed in, so it is possible to create multiple
  callbacks for the same listener
- However, when removing the listener, only the latest
  entry is removed, which could result in improper cleanup
- If a listener is already in the map, do not add the listener
  with the new callback

Change-Id: I46f344eb55db38916224f7765cd838347ac8f0f2
Bug: 168886894