Fix an incorrectly created race accessing Wifi Supplicant

ScanModeState is trying to undo whatever it did in its enter(), in its
exit() function. But doing that is incorrect because it is possible to
transition to multiple states that require different conditions.

In this bug, the state machine transitioned from ScanModeState to
WaitForP2pDisabled state; in response to Stop Supplicant command. Well,
when we are trying to stop supplicant, there is clearly no need to
enable P2P or load all networks. But since this code exists in exit(),
it is executed nonetheless, causing race conditions accessing the
wpa_supplicant (WifiStateMachine is trying to shut it down, but
P2pStateMachine is trying to bring up the p2p interfaces).

We solve that problem by moving this code to the place where we transition
to DisconnectedState - since that's the state that needs this as a
precondition.

Bug: 10761752
Change-Id: Iaf0ffd8056de8533b5d2bfdf8c440fbb7e406dac