The `PreloadMediaPeriod.selectTracksForPreloading` can be called for multiple times at the preloading stage (before the period is being played). For example, when the period resumes preloading. This change fix the assertion failure in `ProgressiveMediaPeriod.selectTracks` caused by the wrong implementation of `PreloadMediaPeriod.selectTracksForPreloading` when it is trying to retain the previously preloaded streams.
Also the `TrackSelectorResult` parameter is changed to a list of `ExoTrackSelection`. We should compare the selections only rather than considering the `RendererConfiguration` in the `TrackSelectorResult` to decide whether to retain the streams, as for preloading case the renderers haven't consumed the samples yet.
PiperOrigin-RevId: 609126868
d952a06214