* Skips setting undefined device id to sink in audio preview.

* Fallbacks to use labels for user selected devices.

* Fixes comment.

The state about currently opened devices is filtered and not stored, where we only store when user selects a device preferences.
Also allow changing input devices for Firefox when we are not in a conference.

Searching for a device (id) by label alone can result in
false results when devices share labels, such as a mic
and speaker having the same label. To prevent such,
specify the device kind to be found instead of iterating
over all device kinds.