refactor for async dao

[pca006132]

Depends on #267. I split the changes into commits, so it should be slightly simpler to read. That said, I am not sure if the commits are independently compilable. I did not spend time on that.

This is mainly for the async dao, which should improve download performance; at least things should not crash. That said, I also changed things like the API for downloading songs, since I noticed a lot of duplication there.

Note that I haven't yet tested this much, cuz this is rebased from elsewhere. Will need some time to do that.

Also, AutomotiveRepository is not fixed. I don't see an easy way to make this async:

tempus/app/src/tempus/java/com/cappielloantonio/tempo/service/MediaLibraryServiceCallback.kt

Lines 364 to 374 in 6698052

	override fun onAddMediaItems(
	mediaSession: MediaSession,
	controller: MediaSession.ControllerInfo,
	mediaItems: List<MediaItem>
	): ListenableFuture<List<MediaItem>> {
	return super.onAddMediaItems(
	mediaSession,
	controller,
	MediaBrowserTree.getItems(mediaItems)
	)
	}