Shave off the number of calls in video-preview and video-provider by
using a stream storage
We don´t call an upfront gUM in video-preview anymore to lift the
fingerprinting barrier on device labels and IDs. Flow has been reversed:
upfront enumerate, load first preview, then check if previous
enumeration was obfuscated.
Add a stream storage in video-preview`s service to avoid re-fetching
them in video-providerj
Remove some unneeded video-preview container props
Improve some of video-preview`s error locales
Some browsers seem to (occasionally) not return the getUserMedia promise call and the
user gets stuck in this state unable to share her/his webcam.
Since enumerateDevices still works even on a gUM rejection this includes a racing
timeout that skips gUM. Configured at settings `gUMTimeout`.
Reproduced with Windows 10 Chrome 87.
