wifi(implementation): Move out clearing on stop complete

Currently, we invoke WifiLegacyHal.invalidate() inside the
|on_stop_complete_internal_callback| std::function. |invalidate|
nulls out |on_stop_complete_internal_callback| inside it which might be
considered a violation since we're deleting the std::function object
as we're executing it.

Bug: 36095153
Test: Compiles, start/stop wifi multiple times.
Test: Verified that a crash is not seen in asan build.
Change-Id: If83c1cbee78a1b6eca12686fcc1ec1de67134f13