Fix "system default" option for media devices

[johnshaughnessy]

Prepend a System Default option for each MediaDeviceKind (audioinput, audiooutput and videoinput).

Also remove a few unused functions, indirection, and dry up some code where possible.

This fixes a bug where resetting the user preference back to the default in the preferences screen would:

• Enable whichever device is the system default, but
• Show whichever device happens to be first in the list.

[keianhzo]

This looks mostly good. The main thing I see is that we are reverting the System Default setting to the actual default device in some places including the Mic Setup Modal or the Audio Toolbar.

In the preferences screen everything works as expected because we are saving the pref and calling startMicShare with updatePrefs set to false so we don't overwrite the store.

When calling it from other places like useMicrophone, useMicrophoneStatus or ui-root we en up saving the currently used device in the preferences so we are overwriting it:

hubs/src/utils/media-devices-manager.js

Line 182 in c03f521

const micDeviceId = this.deviceIdForMicDeviceLabel(this.micLabelForAudioTrack(this.audioTrack));

We have to save in preferences the currently used deviceId not the one coming from the audio track. Also we probably need to update selectedMicDeviceId to don't return the audio track based label but the currently stored deviceId similarly to how we do it in selectedSpeakersDeviceId.

[keianhzo]

Thinking about this again, I think the best approach is not move the mic setting responsibility to the media devices manager and just update the preferences from anywhere else. I've made a proposal in this fork: eec7740 Feel free to merge that commit or disregard if you don't feel it's the right thing.