This is necessary for the child cl that `TrackSelection`
in two distinct class. It avoids to split the array
version of such class too.
TrackSelectionArray exist to have an immutable array of TrackSelection.
Internal users are trusted to not mutate the array.
One drawback of this approach is that a `TrackSelectionArray`
has to be allocated on the boundary of the `Player` interface.
This should not be a performance issue as this only happens
on trackSelection changes, when the user calls
`Player.getCurrentTrackSelections` and on
`updateLoadControlTrackSelection`.
#player-to-common
PiperOrigin-RevId: 353582654
bf3816bd41