wifi: Add support for multiple chips in IWifi

The modified HIDL interface supports multiple chip instances on the
device. Modify the |IWifi| interface implementation to support the new
methods.

NOTE: The legacy HAL implementation will continue to only expose 1 chip.

While there,
Change the |callbacks_| member to std::vector instead of std::set. We
don't really need a set to store the list of callbacks.

Bug: 32003988
Test: Compiles
Change-Id: I31e704100f716e223095890279bdf93d2a04377e