The `PreloadMediaSource` has below two new public methods that suppose to be called by the app:
* `preload(long)` allows the apps to preload the source at the passed start position before playback. The preload efforts include preparing the source for a `Timeline`, creating and caching a `MediaPeriod`, preparing the period, selecting tracks on the period and continuing loading the data on the period.
* `releasePreloadMediaSource()` allows the apps to release the preloaded progress.
The `PreloadMediaPeriod` is designed to facilitate the `PreloadMediaSource` for the preloading work. It has a new package-private method `selectTracksForPreloading` which will cache the `SampleStream` that corresponds to the track selection made during the preloading, and when the `PreloadMediaPeriod.selectTracks` is called for playback, it will uses the preloaded streams if the new selection is equal to the selection made during the preloading.
Also add a shortform demo module to demo the usage of `PreloadMediaSource` with the short-form content use case.
PiperOrigin-RevId: 574439529
