Migrate WebSockets to new SocketApi from SDK

[nielsvanvelzen]

The new WebSocket code from the SDK is still in beta. The API should be finished so no changes are expected in the app. Some issues in de SDK right now include unnecessary reconnects on subscription changes and the lack of automatic reconnection logic. These will be fixed before SDK 1.2.0 is released as stable and app 0.14 is released in beta/stable. The current issues are minor and shouldn't prevent us contributors from using the master branch on our devices.

Changes

• Add new SocketHandler to replace the TvApiEventListener
  • Used the new WebSocket code from the SDK
  • Fully rewritten in Kotlin, more readable and less code now
  • Doesn't support screen mirroring as the ItemLauncher does not support the SDK
• Add endPlayback overload that can close the activity
  • It's a hack, but we don't want to keep track of activities in the SocketHandler
  • Still way better then it was before
• Move session capability announcement from ApiBinder to new SocketHandler
• Remove currentActivity stuff from TvApp.java

Documentation for the WebSocket API is available here: https://github.com/jellyfin/jellyfin-sdk-kotlin/blob/master/docs/websockets.md

Issues

[DavidFair]

Some general design points whilst this is a draft PR (so I'm hoping open to change).
I'm looking forward as the project gradually migrates to the new SDK though, nice work 👍