Don't leak captive portal broadcast receivers.

Since aosp/937885, mLaunchCaptivePortalAppBroadcastReceiver is
nulled out, but not unregistered, when exiting MaybeNotifyState.
This causes the BroadcastReceiver to be leaked because it is no
longer unregistered when processing CMD_NETWORK_DISCONNECTED.

Fix this by unregistering the receiver in MaybeNotifyState#exit.
Also remove the unregister call in CMD_NETWORK_DISCONNECTED: it
is no longer necessary, because StateMachine, before quitting,
will call the exit method of the current state and all its
parent states.

Bug: 122164725
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: I447b3cfd3821dda19482ed962dd59ec61fc5fc9b