[windows] QML media player always uses audio stream, even when there is no media playing

Tested with 6.5.2 and 6.6.1 using both ffmpeg backend and windows backend.

Whenever we create a qml video player, an audio device will be in use, even if there is no video playing. The QML video player will hold on to the audio component, even if you use a loader to unload it. (set its src component to null.)

You can test this by opening the qml Multimedia Player sample without any other apps running: (https://doc.qt.io/qt-6/qtmultimedia-video-mediaplayer-example.html) without opening a video, open cmd as admin, type powercfg /requests press enter.

You will see something like this:

SYSTEM:
[DRIVER] Realtek High Definition Audio (HDAUDIO\FUNC_01&VEN_10EC&DEV_0892&SUBSYS_14629C56&REV_1003\5&bce21bb&0&0001)
An audio stream is currently in use.

close the multimedia player. type powercfg /requests press enter. you will see the following

Impact: Constantly using the audio device will prevent windows from automatically entering sleep mode (s3)

expected fix: when there is no active video playing/the video is muted/the page is unloaded etc. stop using the audio device.