Fix DHCP handling at disconnect/reconnect

Wifi can have a quick disconnection followed by a reconnection. We used to
create a new DHCP state machine thread for every new connection and
never really waited until it quit after disconnect. This may have lead to
situations where repeated disconnect/reconnects resulted in multiple dhcp
start calls.

We now keep the statemachine after a disconnect and only shut it at supplicant
stop.

Bug: 6417686
Change-Id: Icf66efdc654be886e3eb46c81f09f8cce536f2f6