async unloadAndSavePreload(
      initializeMediaSource = true, keepAdManager = false) {
    const preloadManager = await this.savePreload_();
    await this.unload(initializeMediaSource, keepAdManager);
    return preloadManager;
  }
Unloads the currently playing stream, if any, and returns a PreloadManager
that contains the loaded manifest of that asset, if any.
Allows for the asset to be re-loaded by this player faster, in the future.
When in src= mode, this unloads but does not make a PreloadManager.

@param {boolean=} initializeMediaSource
@param {boolean=} keepAdManager
@return {!Promise<?shaka.media.PreloadManager>}
@export