aout: winstore: select the audio output using ActivateAudioInterfaceAsync

See the API https://docs.microsoft.com/en-us/windows/win32/api/mmdeviceapi/nf-mmdeviceapi-activateaudiointerfaceasync

It requires a recent mingw-w64 with the added API.

We request the IAudioClient asynchronously and return the found client or NULL
once the async call as completed.
The code originates from the vlc-winrt project with some modifications.

Do not rely anymore on the local "winstore-client" variable to cache the
IAudioClient. A client is queried/used between each Start/Stop calls.