Add endFile calllback to EventObserver

[tymmesyde]

Add mpv_event_end_file to the EventObserver with reason and error code
Let me know if you think this is fine to add and if it needs some changes

private fun eventObserver() = object : MPVLib.EventObserver {
   ...
   override fun endFile(reason: Int, error: Int) {
        when (reason) {
            MPV_END_FILE_REASON_ERROR -> Log.e("Player errored: ${error}")
            else -> Log.d("Player ended")
        }
    }
}