QPlatformMediaDevices is accessed before main on Android

On Android, QPlatformMediaDevices is created before main starts executing. This happens because the onAudioInputDevicesUpdated function in QAndroidmediaDevices  is called early at startup, and initializes the QPlatformMediaDevices singleton.

This is a design problem because we end up connecting signals before QCoreApplication has been created, and we don't have a reliable mechanism to make sure events are processed at the expected time.

This prevents implementing fix for QTBUG-121768 and makes tst_qmediadevices more complex than it should be.