wifi: Make methods deliver status synchronously (1/3)

Make the following |IWifi| HIDL interface methods return a synchronous
status code:
a) start()
b) stop()
The other methods in this interface do not have a failure case and hence
not returning a status code.

This changes the nature of event callbacks registered for each
interface. Previously, every operation's status was sent to all the
registered event callbacks. Now, only the caller is notified of the
operation's status via the passed synchronous callbacks. The event
callbacks are now used to broadcast only important state changes/events.

Bug: 32056230
Bug: 32061909
Test: Compiles
Change-Id: I95dc3fa139ac3ac7500d81e9e0dbc4f4de04e127