async detachAndSavePreload(keepAdManager = false, saveLivePosition = false) {
    const preloadManager = await this.savePreload_(saveLivePosition);
    await this.detach(keepAdManager);
    return preloadManager;
  }
Detach the player from the current media element, 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 detach but does not make a PreloadManager.
Leaves the player in a state where it cannot play media, until it has been
attached to something else.

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