Screen streams were only deemed unhealthy when the transport's ICE state
transitioned to failed. That was as good as nothing because the stream would
stay frozen with no visual UI feedback until it reconnected. Bad UX.
This commit addresses that issue via two changes:
- A stream is deemed *potentially* unhealthy now if the transport's
state becomes disconnected
- If a stream is deemed potentially unhealthy, a monitor probe is
started to check whether there is media/packet flow (every 500ms).
If there's no packet flow, the stream is flagged is factually unhealthy and
UI feedback about that is rendered.
It's still not as good as it could be - relying on disconnected still
leaves a couple of seconds of silence to be dealt with. For that to be
addressed the prober would have to run nonstop, but that's for later.
d350afd194