[Cast MRP] Do not call chrome.cast.channel.open / close in CastChannelService
for in-browser Cast discovery

- Passed shouldOpenChannel flag to CastChannelService.init()
- Added CastChannelService.addCastChannel(), which adds an always open cast
channel to service without actually opening it
- Skip open / close cast channel if shouldOpenChannel flag is false

PiperOrigin-RevId: 169416458
1 file changed