PreviewVideoActivity: improve button UX
Restore fast forward/rewind buttons in fullscreen video
PreviewVideoActivity: convert to Kotlin
PreviewMediaFragment: fix restoring seek position when returning from fullscreen stream
PreviewVideoActivity: fix control buttons
PreviewVideoActivity: pause player when activity stops
PreviewVideoActivity: ensure player is stopped when closing fullscreen
PreviewMediaFragment: stop video when stopping activity, and don't recreate existing player
