`TrackSelection` had mutation methods which were to be called only
internally by ExoPlayer components but were exposed in the
public `Player` interface.
The mutation methods have been moved out of `TrackSelection`
to a new class `ExoTrackSelection`.
Current track related read-only method have also been moved out,
because they are actually something quite unclear.
Even for a single item playlist, it's the track being buffered rather
than the track being played, which is unclear.
But when you have a playlist it starts to get really confusing,
because if the next item is being buffered, then it's actually
the last track to be buffered in the currently playing item.
As a final aside, the implementations don't do proper thread synchronization
to ensure visibility of updated state by the calling thread.
Exposing those mutable methods in the public `Player` interface
was problematic because they leaking internal concepts of `ExoPlayer`.
This is also required to minimize the `Player` interface for long term
stability.
`ExoTrackSelection` is a subclass of `TrackSelection`.
This is not ideal as an `TrackSelection` implementation could
break the current immutability.
This was done in order for this refactor to be simpler.
A future patch will fully split the two classes.
All `MediaPeriod` and `Sources` had to be updated to use the new
`TrackSelection` dynamic aspect class name.
An alternative would have been to break ExoPlayer's public API, keeping
`TrackSelection` as the dynamic aspect name, and calling the public static
aspect class `TrackSelectionState` or similar.
Nevertheless, while it would have impacted less files, it would have
many more small apps and casual users of ExoPlayer.
#player-to-common
PiperOrigin-RevId: 353637924
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