iOS 15: playing over AirPlay doesn't go to the next episode

Similar to #47 but it affects only iOS 14 and `DefaultPlayer`.

## To reproduce

1. Device needs to be running iOS 15
1. Do a clean install of the app
1. Add at least two items to the Up Next queue
1. Play the first and lock the phone
1. Change the source to play over AirPlay
1. Skip to the last 20 seconds or so (from the lock screen)
1. Wait for the episode to end
1. The episode will end and nothing happens, triggering the next episode never happens¶

¶ It worths nothing that sometimes it works just as intended, but most of the time not. Also, if the episode is downloaded this never happen. It seems that is something related to `DefaultPlayer`.

Start in the DefaultPlayer playback and queue-advancement flow, comparing AirPlay streaming with downloaded playback. Reproduce the listed iOS 15 steps with two queued episodes and verify that the next episode starts reliably when the AirPlay episode ends.

Written by the indexing model from the issue text.