Fix handling in DisconnectingState

Now that we also handle supplicant state change to identify that
a network is disconnected (in case CTRL-EVENT-DISCONNECTED goes missing),
it is dangerous to defer all supplicant state change messages
while in DisconnectingState.

It may happen that the CTRL-EVENT-DISCONNECTED goes missing while in
DisconnectingState resulting in a supplicant state change of 0 (disconnect)
getting deferred.

Eventually after a connection completes, the supplicant state change events
get handled and the state machine goes into DisconnectedState.

Fix by having state machine switch out of DisconnectingState once we
do not immediately see a CTRL-EVENT-DISCONNECTED state.

Bug: 5490789
Change-Id: Ia2263795e53c51da0a2bfeefecfeb6256d6c5267