Loading custom MediaProvider in playlist doesn't work

[pablo]

Steps to reproduce:

1. Create playlist containing items with mixed MediaProviders
2. Include an item in the playlist with a loaded MediaProvider (not the first item)
3. Play a playlist item with a built-in MediaProvider
4. Switch to item with the custom MediaProvider
5. The player will attempt to load the item in the last known MediaProvider

[pablo]

Fixed in 919

[pablo]

Duplicate of 814