Originally a `PlayerId` has to be passed and it should be the same id of the player who is going to play the sources, but it turns out to be unnecessary.
When preloading, we can set a `PlayerId.UNSET` inside of the `PreloadMediaSource`, as there is no ongoing playback. And when the source is handed over to player, player will set the player's `PlayerId`.
PiperOrigin-RevId: 597475119
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
