Fix driver start/stop handling

The driver start and stop events will be removed as they are now
redundant (and incorrect)

Move to using supplicant INTERFACE_DISABLED indication for deciding
if driver has started or stopped

When driver start is given, the supplicant will first switch to
INTERFACE_DISABLED state and then out of it to start connecting.
We should use this indication to find out driver has started

When driver stop is given, the supplicant will just switch to
INTERFACE_DISABLED state and this indicates driver has stopped

Bug: 4550125
Change-Id: I242cda97951a733c8707c8d99cdfcc9a33b75c2d