It's currently very difficult to actually set a reason for track selection
triggered by a SelectionOverride. It would require creating a new custom
TrackSelection.Factory where the reason gets injected somehow before the
override is specified on the player.
Simplify this whole procedure by allowing to set the reason directly and
forward this reason to all fixed selections created based on this override.
PiperOrigin-RevId: 240114942
b0bb6ec8a5