fix: improve exoplayer scoping

[Pittvandewitt]

This PR splits the PlayerViewModel, so that each fragment or activity where an exoplayer is used has its own properly scoped exoplayer. The result is that an exoplayer is created when the viewmodel of a fragment that needs a player is created and destroyed when the viewmodel is. The exoplayer is no longer recreated upon orientation change.
This means that media keeps playing without interruptions upon orientation change, fixing #5754 (comment)

The exoplayer no longer lives inside the player fragment or in the activity scoped viewmodel.

[Bnyro]

I don't feel any difference when rotating the screen honestly, but it's reasonable to move the player ownership to the view model.

Thanks!

[Pittvandewitt]

I don't feel any difference when rotating the screen honestly, but it's reasonable to move the player ownership to the view model.

Thanks!

I don't feel anything in the update in which this PR is merged either. I'll go back to the drawing board when I got some spare time again to find out why.