TracksInfo is very similar to
`MappingTrackSelector.MappedTracksInfo` with some
fields removed to simplify the Player API,
notably it doesn't expose the renderer concept.
A significant difference is the addition of a `selected` boolean
field which avoids having a separate `getCurrentTrackSelection`
API.
This cl is a part of the bigger track selection change,
splitted for ease of review.
In particular, the MediaSession implementation and UI usage
have been slitted in child cls.
Find all cls with the tag:
#player-track-selection
PiperOrigin-RevId: 400937124
ac881be2fc