QAudioInput: Handle assignment of devices that have been disconnected

It's currently possible for the audio set a device that has been disconnected using the following steps:

1. Load all audio-inputs using QMediaDevices::audioInputs()
2. Store one of the devices in a variable
3. Disconnect that device
4. Pass the stored variable into QAudioInput

Currently we don't really handle what happens in this scenario. Since we have reconnect-recovery functionality already, I would argue we should accept this value but not activate the device until it is connected.

This change should also include a small change in the documentation to make it clear what happens to the user.