fix: set setScreenShareEnabled when detect replaykit state changed

[lambiengcode]

I found that the example app is missing the active Broadcast Extension tracking or is disabled. This PR fix the #242 and #392.

I'll set setScreenShareEnabled is true if detect function broadcastStarted() called in SampleHandler and isScreenShareEnabled() == false or set setScreenShareEnabled is false if detect function broadcastFinished() called in SampleHandler and isScreenShareEnabled() == false

_replayKitChannel.setMethodCallHandler((call) async {
      if (call.method == 'closeReplayKitFromNative') {
        if (!(room.localParticipant?.isScreenShareEnabled() ?? false)) return;

        await room.localParticipant?.setScreenShareEnabled(false);
      } else if (call.method == 'hasSampleBroadcast') {
        if (room.localParticipant?.isScreenShareEnabled() ?? true) return;

        await room.localParticipant?.setScreenShareEnabled(true);
      }
    });

[CLAassistant]

All committers have signed the CLA.

[cloudwebrtc]

lgtm

[patrikheinonen]

Is it possible to add something about this to the readme.md @cloudwebrtc

[cloudwebrtc]

Will create PR for README.md later