Remote screenshare support

[thyal]

This adds support for rendering remote screenshares, with a new property on the state object, containing a list of active screenshares. This allows you to render screenshares separately from normal videos:

 const { remoteParticipants, screenshares } = state;
 
       {remoteParticipants.map((p) => (
        <VideoView key={p.id} stream={p.stream} />
      ))}

      {screenshares.map((s) => (
        <VideoView key={s.id} stream={s.stream} />
      ))}

The screenshares are also added as a stream in the streams array of the remote participant - and there's some cleanup to the addStream functionality. This fixes the bug where we render the screenshare as a participant stream if you join a room with the SDK where a screenshare is already active.

Local screenshare (render/start/stop) is not handled in this PR, it will come in a follow-up.

Screenshares are currently shown twice - this is because of a known bug with the event listeners.

Test plan

1. Add the relevant code to you SDK test app - as shown above
2. Join a room with an sdk client and a remote participant
3. Start a screenshare with the remote participant, you should see the screenshare rendered in the SDK app
4. Leave and join again with the SDK app - you should see both the video and screenshare view from the remote participant
5. Stop screenshare, it should go away and the video of the remote participant should stay.

[nandito]

It works pretty well, I only found 1 thing that should be fixed before merging.