Persist archive playback progress

I wrote some preliminary code for this in KdvsPlaybackPreparer: prepareBroadcast().

If a user left their last open process of our app with a broadcast archive prepared, we'll want to re-initialize the player to the same exoPlayer position.

I'm not entirely sure how viable this will be for an archive which the user has not downloaded, as the exoPlayer position is actually based off a virtual window, and a stream wrapper would likely make this task difficult.