Fix a race in WifiNative objects

disconnectFromSupplicant doesn't have any synchronization;
so it can cause problems when WifiStateMachine is disconnecting
and P2p statemachine is sending commands.

Bug: 22667667
Change-Id: I92e6ee44618813341e171f84442791a6403a5010
(cherry picked from commit b577f391af2c484e443c19b3df1d62cc0924692a)