Add onDisconnect callback in root radio connection class.

From CDMA/GSM and IMS connections, notify listeners of the disconnects.
This is used as a replacement for the traditional phone.notifyDisconnect
method.  This ensures that a more reliable direct path is used to notify
of disconnects in the underlying connections.

Test: Manual
Bug: 70389936
Change-Id: Id1809bc9deb7675e94341664dc08707dc81b304c